Japan uses 110v and has a different adapter hole standard than in Europe.
Most people do buy an good travel adapter before going away, but what if you forgot, or if your current adapter breaks during the stay?. Fear not, adapters for European to Japan are available at least at the bigger electronical chains. I bought my adapters at Yodobashi in Kyoto (just walk a hundred meter north when leaving the main entrance, it’s on the left side where you see an mcshit– sorry, mcdonalds on the right).
I bought two slightly different models, (one with slightly tighter plug holes that were perfect for my mac adapter).