Japan’s prohibited list is short, and it ignores what is legal where you live.
Narcotics
Prohibited narcotics include heroin, cocaine, MDMA, opium, cannabis, stimulants and psychotropic substances.
Cannabis sits on the list beside heroin and cocaine. Japan makes no exception for places where it is legal, decriminalised or medically prescribed. Travellers from parts of the United States, Canada and Europe are the most likely to get this wrong, because at home it does not feel like a serious question.
Weapons and explosives
Pistols, other firearms, their ammunition and pistol parts are prohibited. A component counts the same as the whole.
Explosives such as dynamite and gunpowder are also prohibited, as are precursor materials for chemical weapons and germs likely to be used for bio-terrorism.
Money and documents
Counterfeit, altered or imitation currency, bank notes and securities are prohibited, as are forged credit cards.
Content and copies
Obscene or immoral materials such as pornography are prohibited.
So are articles that infringe intellectual property rights. The souvenir market in a neighbouring country is a common source, and a convincing fake handbag is a prohibited article, not a bargain.
If you are not sure
Everyone entering Japan submits a customs declaration, and baggage may be inspected whether you declared electronically or on paper. If something you carry might fall into one of these categories, raise it on the declaration, before the inspection.
The full official list is linked below.