Argynnis paphia looks very similar to Argyronome ruslana which also occurs in Japan.
However, the underside of the wings of the two species are quite different.
Argynnis paphia features distinct silvery-white bands over a greenish background, while Argyronome ruslana lacks these pronounced, unbroken silver bands.

If you only have a photo of the upperside of the wings (like my photos), it is much more difficult to identify.
The specific subspecies occuring in Japan are Argynnis paphia geisha (which is a little different from the European form), and Argyronome ruslana lysippe.
Regarding these two (sub)species in Japan, it seems that the dark-brown part of the U-shaped spot (on the forewings) left and right from the head of the butterfly, is rather thin in Argyronome ruslana en much thicker in Argynnis paphia.
Furthermore, regarding the two species occuring in Japan, in Argyronome ruslana, the 4th brown spot from the tip of the forewings is distinctly smaller than the other spots next to it, whereas in Argynnis paphia they all are more or less the same size.

Shirakawago, Gifu Prefecture, Japan, 26 September 2018