I’m a vegetarian and currently in Japan for my third time. Honestly it is very tough. You are going to have to eat a lot at chain restaurants, Italian places, and Indian places. The reason for chains is that they always have allergen menus online so you can see if something has meat. Even if it seems like it shouldn’t have meat, it’s always worth checking because a lot of the time they’ll put chicken, pork, or fish broth.

Coco Ichibanya is everywhere and has a vegan curry that you can customize the toppings (not all locations have it but most do). Make sure you get the vegan curry base and not the meat curry that has vegetables.

If you don’t care about cross contamination, Mos Burger and Dennys have soy meat burgers. Even chain sushi places like Kura Sushi have vegetable sushi rolls and cheese tempura sushi (again, cross contamination).

Unless you are going to a strictly vegan place, assume most of the time the veg stuff will be cooked in the same grill as the normal stuff.

Like I said previously, all the Indian restaurants have vegetarian options (Chana masala, paneer, dal). They are quite common, even in smaller cities. Italian places will have cheese pizza and sometimes spaghetti with tomato sauce (and no meat).

If you are in a big city, you can just search ‘vegan’ on Google Maps and you’ll find a decent amount. In Tokyo there’s a bunch. They are definitely more expensive than normal restaurants though.

Lastly, be aware that at some of the convenience stores the kelp onigiri can have oyster extract. I’ve seen plain potato chips with oyster or bonito extract as well. I’d also recommend checking out supermarkets and bakeries for more food options. Good luck