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These are also all over the place during summer months here in Japan! You can see their shells all over the place.
Hi realjapanesegreentea,
I think your snail is Euhadra peliomphala, but I am not 100% sure. Wikipedia says that the species "exhibits exceptional geographical variation". In inaturalist, there are many variations shown under this species' name. So, I think it might help if you would like to take a look at them. I hope that my ID Suggestion is correct because, according to Wikipedia, Euhadra peliomphala is endemic to Japan. Nice spotting!