Punishing those who are only experimenting with a drug and who would mostly grow out of it very soon is counterproductive, especially when the direct health risks of the drug are as moderate as in the case of marijuana. It is questionable if legalisation would lead to any significant increase in use. This is not the case. Use rates amongst the Dutch are similar to rates amongst Germans. That's because the Dutch spend more of their drug policy money on health-care based policies that work and not on police and prisons that are ineffective.
Drug use is affected far more by social attitudes, fashions and information than by laws.
Japanese marijuana seizures increased six-fold from 1998 to 1999. Drug laws don't control use, they react to it. The end of alcohol prohibition in the USA was preceeded by 13 years of rising illegal alcohol use. It is likely that before marijuana is legalised or tolerated in Japan again its use would already have stabilized at the rate at which it which it will be consumed without criminal penalties. A drug policy that keeps marijuana illegal can not talk honestly about the risks of nicotine and alcohol , for fear of either showing the risks from marijuana as benign by comparison or exposing its own hypocrisy.