The first large scale shipment of cannabis hidden in commercial freight in a shipping container was only intercepted in October 1998. The ones getting caught generally are either couriers at airports or users selling to their friends who get jumped on because they screw up some other way. Like the reporter who staged a robbery while on amphetamines or the lamp grower in Akita who got busted because he messed around with a 15 year old girl he met through a date club. Since most marijuana users are otherwise law-abiding citizens that makes it very difficult to quote any ratio between who does what and who gets caught. The most accurate indicator of use are probably attitudes expressed in polls, since attitudes reflect, precondition or prevent experience with drugs.
Users can voice support for legalisation without having to admit to illegal activity.
This may be the sort of magnitude of illicit drug culture, tasters, casual and habitual users, up to about 3-4 million Japanese. How many are cannabis users and how many use other drugs is hard to say. That would be 100,000-700,000 cannabis users, about one tenth of the percentage in other industrialized countries even at the high end of the range. If marijuana had the same detection rate as amphetamines then one would expect about 75,000-150,000 users, or roughly 1 in a thousand. It is unlikely that there are much fewer users than that.