I asked myself the same question before traveling abroad because, when you're asthmatic, Ventolin (salbutamol) is one of those things you always prefer to have with you.
To my knowledge, salbutamol used in inhaler form to treat asthma isn't considered a drug in the usual sense, but each country may have its own rules regarding the importation of medications. The best thing is to travel with your inhaler in its original packaging, along with the prescription or a medical document indicating the treatment and the active ingredient used.
I also looked into how Ventolin works, its role, and the precautions related to its use via this guide:
https://ventoline-salbutamol.mystrikingly.com/For a trip to Japan, I’d especially recommend checking the official rules before departure and not waiting until you're there to look for an equivalent. Even if similar treatments probably exist, it’s more reassuring to have your usual treatment with you, especially in case of an attack or environmental changes (pollution, allergies, fatigue, jet lag…).
Personally, I always prefer to plan a little safety margin: keep the inhaler in my carry-on luggage, have a copy of the prescription, and know the name of the active ingredient (salbutamol) in case I need to explain the treatment.