It is currently legal to use electronic cigarette devices (vapes) in Japan, however, nicotine is considered a toxin regulated by the Ministry of Health, Labour, and Welfare, and so vape shops in Japan are not permitted to sell ecigarette juice containing nicotine.
In the past, the most common solution for domestic Japanese vapers seeking nicotine liquid was to import it from the USA, but in October 2021 it became no longer permissible to send vape products through the mail in the USA (and by extension, export them to Japan).
Currently, the most common solution for vapers in Japan is to self-import nicotine-containing eliquid, which is permitted in small amounts of up to 120 ml at a time, or more commonly, nicotine salt liquid and DIY it.
What does the law say?
The official documentation on importation of nicotine into Japan for personal use can be found in this MHLW PDF in the Q&A section:
法律上、電子たばこ用のカートリッジ及びリキッド(いずれもニコチンを含有 するもの。以下同じ。)は医薬品に該当します。税関限りの確認で通関が可能な 数量は、用法用量からみて１ヶ月分(タバコ 1,200 本分又は吸入回数 12,000 回 分。カートリッジの場合は 60 個、リキッドの場合は 120ml。)とし、１ヶ月分を 超えてカートリッジやリキッドを個人輸入する場合は、薬監証明の取得が必要 です。同一の貨物の中に、カートリッジとリキッドが混在している場合には、 それらを合算した数量を基準とします。
Or, in English:
Q63- Are electronic cigarettes subject to regulation under the law? What is the method for importing them?
By law, cartridges and liquids for e-cigarettes (both containing nicotine; the same shall apply hereinafter) are classified as pharmaceutical products. The quantity that can be cleared by customs only is one month’s worth (1,200 cigarettes or 12,000 inhalations, 60 for cartridges, 120 ml for liquids) in terms of dosage. If you want to personally import cartridges or liquids in excess of the above, you need to obtain a drug supervision certificate. If cartridges and liquids are mixed in the same cargo, the total quantity will be used as the standard.
Where can you import vape supplies from?
The most common method is now DIY, and the supplies are easy enough to buy and import, consider the following starter-pack of links: