Japanese often refer to their country sarcastically as a ''paradise for spies.''
One leading magazine recently complained that the country was becoming a ''cockpit for spooks of varied stripes and feathers.''
But Japanese security authorities are seriously handicapped in their battle with the spies by the absence of any anti-espionage law. Thus, a private citizen obtaining state secrets and passing them on to a foreign power can be charged only with theft. And even a senior military officer caught in 1980 passing classified information to the Soviet embassy could be sentenced only to a maximum one-year jail term.
The climate in favor of stronger measures against spies could be helped by the US release earlier this month of congressional testimony given by Soviet defector Stanislav Levchenko, who confessed that his four-year assignment to Tokyo in the late 1970s as a magazine correspondent was a cover for his real activities as a KGB field case officer.
There has been the usual guessing game as to the identity of the 200 Japanese agents Mr. Levchenko said the KGB had used, including government and opposition politicians and journalists. Some of those who believe they are the ones he has in mind have been quick to issue denials.
Police say they will take no action on the Russian's claims, and, after the initial shock, many Japanese seem to be more interested in why the United States chose this particular moment to release Levchenko's testimony made last July. The most popular theory is that Washington wants to shock Japan into being more aware of the espionage danger, especially from the Soviet Union, as part of the campaign to encourage more Japanese defense spending.
Experts say the Soviets, North and South Koreans, and to a lesser extent the Chinese and Americans, currently are most active in the intelligence field.
Intelligence sources claim there are probably at least 200 Soviet intelligence agents in Japan, including diplomats, representatives of the Aeroflot airline, Intourist travel agency, a shipping line, and correspondents of various Soviet media.
One function is to influence Japanese public and political opinion, to make it more pro-Soviet, dropping claims for return of Soviet-held northern islands and signing a formal peace treaty, plus more generosity in trade and economic aid.
The other side is straight spying. Naturally the Russians want to know more about US-Japan defense cooperation as well as Japan's diplomatic contacts with such countries as China.
But the public security investigation agency, the main antisubversion organization, is believed to be more concerned at present by Soviet business espionage.
This has become more pronounced with Japan's emergence as a major force in the electronics and optics industries (for instance, cameras for ''smart'' bombs , lasers for new space-age weapons). The US and Soviet Union, in fact, are competing for the same technology, although Washington hopes to obtain this through the front door.
A major spy battleground is the islands northeast of Hokkaido which the Soviets seized from Japan at the end of World War II and have refused to hand back. Two years ago, Japanese police arrested a number of Hokkaido fishermen said to have provided information to Moscow in return for permission to enter the rich fishing grounds around the islands.
But the information they provided reportedly was not particularly significant - most of it could have been gleaned from newspapers and public documents. The case was further complicated when leading newspapers claimed the fishermen were really double agents, reporting to Japanese authorities on Soviet defense facilities on the islands. Four years ago, a spy scandal erupted over stolen defense agency documents passed to China. Since then, say intelligence sources, the Chinese have been maintaining a low profile.
The same is not true for the North and South Koreans, who appear to conduct more intelligence activities than everyone else combined.
Police and coastal patrols on several occasions have managed to catch North Korean spies being landed by fishing boat on Japan's western coast. Of major interest for both north and south, are the estimated 700,000 ethnic Koreans here , many of them second or third generation Japanese residents. There is an intense effort to win allegiance to either Pyongyang or Seoul, and to recruit spies to be sent eventually to the other side of the demilitarized zone.
American intelligence operations here are reckoned to revolve around promoting pro-American public opinion, as well as keeping tabs on local operations of the ''opposition.'' Some industrial espionage is suspected, but has never been proved.
(The IBM spy case earlier this year shows the Japanese do their share as well. There are several industrial espionage training schools for businessmen here, although their operators insist the emphasis is on defense not offense. Japanese companies, in fact, probably spy more on each other than on foreign rivals these days.)
Japan finds itself under a severe handicap in trying to cope with foreign espionage activities, especially through lack of legal provisions to discourage its citizens from becoming traitors. This stems from the theory that under the present constitution Japan has no ''military'' forces and, therefore, no ''military secrets.''
The only anti-espionage law is one carrying a 10-year prison sentence for anyone disclosing classified defense information of American forces in Japan. Civil servants and military personnel alone can be jailed for up to a year for disclosing other types of classified material.
In the past two years, the ruling Liberal Democratic Party has tried to draft a strong general anti-espionage bill. But with public memories still strong of prewar Japan, where spying was a way of life and the ''thought police'' reigned supreme, the effort has not been well received. Even the Justice Ministry has balked at the idea, citing the many difficulties of defining classified information as well as obtaining a conviction without disclosing national secrets in court.