You are bound by Japanese local laws and subject to the penalties accordingly while in Japan. Be aware that some of the local laws and penalties may seem to be harsher than the standards of your country.
All students are urged to observe Japanese drug laws, which are rigorously and strictly enforced. The possession, receipt, use and sale of drugs, including marijuana, is subject to investigation by the Japanese customs and police authorities. Any student convicted of these crimes will be expelled from our program immediately.
Persons arrested in Japan, even for a minor offence, may be held in detention without bail for two to three months during the investigation and legal proceedings. Further, penalties for drug offences include lengthy imprisonment. Use or possession of small amounts of drugs may result in a prison sentence of between one and ten years and Japanese law allows for a fine of up to 5,000,000 yen. Some people convicted of drug related offences have been issued a deportation order. People deported under these circumstances are not permitted to return to Japan for five to ten years.
You should also be aware that Japanese authorities aggressively pursue drug smugglers with sophisticated detection equipment, drug-detecting dog, and other methods. Incoming and outgoing mail, as well as international packages sent via DHL or FEDEX, is also checked carefully. The Japanese police make arrests for even the smallest amounts of illegal drugs.
Regarding the illegal drug, please refer to the following links:
https://www.mhlw.go.jp/content/11120000/001059014.pdf
https://www.mhlw.go.jp/content/11120000/001059015.pdf
https://www.mhlw.go.jp/content/11120000/001059016.pdf
https://www.mhlw.go.jp/content/11120000/001059017.pdf
https://www.mhlw.go.jp/content/11120000/001059018.pdf