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A butterfly of some sort
Beside the road, which was beside the sea
Possibly, though there is often a wide variation in markings even within the same species. There are also a number of sub-species (including a. hippia japonica) & although I have found a list unfortunately I've not been able to find photos on-line.
Are the black smudge marks in the outer white patches significant, no picture of the Aporia hippia show them. But I don't know anything about butterflies, maybe they appear with old age.
Craig, here is a link you may find useful for future spottings. http://www.asahi-net.or.jp/~ak5t-kmn/
Looks a lot like Aporia crataegi which is native to Japan but has darker veins. It's a member of the pieridae family for sure.