Japan’s prohibited list is short. It is also absolute, and it does not care what is legal where you live.
This is the article most worth reading if you are traveling from somewhere with liberal drug laws, because the gap between home and Japan is wider here than almost anywhere else you might fly from.
Narcotics, and the word that matters
Prohibited narcotics include heroin, cocaine, MDMA, opium, cannabis, stimulants and psychotropic substances.
Cannabis appears on that list alongside heroin and cocaine. Japan draws no distinction based on it being legal, decriminalized or medically prescribed in the country you departed from. Travelers arriving from parts of the United States, Canada and Europe are the people most likely to get this wrong, precisely because it does not feel like a serious question at home.
Weapons and explosives
Pistols, other firearms, their ammunition and pistol parts are prohibited.
Explosives such as dynamite and gunpowder are prohibited. So are precursor materials for chemical weapons, and germs likely to be used for bio-terrorism.
“Pistol parts” is worth reading literally. A component is not a lesser item.
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 neighboring country is a common source of exactly this, and a convincing fake handbag is a prohibited article rather than 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 are carrying might fall into one of these categories, the declaration is the moment to raise it, not the inspection.
The full official list is linked below and is short enough to read before you pack.