Checked against UN INCB controlled substance lists and Japan rules.
Diazepam is a psychotropic in Japan and needs no advance permission. Up to one month's supply may be brought in without an import certificate. Carry the prescription.
Valium contains a substance on the UN International Narcotics Control Board lists. Most countries that signed the UN drug conventions expect you to carry a prescription and declare it.
Check the permitted quantity and formulation before you pack. Carry the prescription and original packaging.
Official source for Japan: www.mhlw.go.jp
Original labelled packaging, the prescription, and a letter naming the active ingredient in generic form — control lists are written in generic names, not brands. What the letter must say. And the destination’s own procedure.
Not legal or medical advice. Rules change and depend on dose, form and quantity. Always confirm with the Japan embassy or health authority before you fly. We never tell you a medicine is "safe" to carry.