What is appropriate clothing to wear to visit a jinja?
This is a question that causes me repeated trouble, both in writing advice for my own publications and in the material I prepare with Jinja Honchō. It is also a question that recently made the international news, when Yasukuni Jinja (靖國神社) banned cosplay and military uniforms that the wearer was not entitled to by virtue of being in service.
I do not know whether Yasukuni has a problem with people coming dressed as Sailor Moon, but they do have a known problem with far-right groups coming in military or pseudo-military uniforms, marching in, and then doing stuff in front of the prayer hall. The double ban covers both actual military uniforms, and things that look like military uniforms but aren’t, because the latter can be described as cosplay.
It is worth noting that at least one jinja in Japan holds events at which cosplayers can come and take photographs in the precincts, and so cosplay at jinja is clearly not a problem per se.
Something similar can be said about exposed skin. The only other jinja I know with an explicit prohibition on certain forms of dress is Shirahama (“White Beach”) Jinja (伊豆白濱神社) in Shimoda, which has a sign telling people not to visit the jinja in swimming costumes — because it is right next to a white beach, which is an official sea-swimming site in summer. However, most jinja frown on excessive exposed skin, and will actually refuse a request for a formal prayer if your clothes are too skimpy.
On the other hand, there are a number of Shinto matsuri where men run around in loincloths, including the sacred purification rituals formally taught by Jinja Honchō. So exposure per se is not a problem either.
I think Yasukuni’s explanation of the reason for their prohibition captures the reason quite well. They say that they want everyone to be able to pay their respects calmly at the jinja. A group of right-wingers in pseudo-military uniforms would, indeed, make that difficult for most people, who would be nervous that there would be trouble.
This also explains the exposure rules. Most people would be made uncomfortable by someone paying their respects in a swimming costume. On the other hand, a group conducting a traditional ritual do not have that effect, even if they are no more covered than they would be if they were in speedos.
Thus, when visiting a jinja, you should dress in a way that will not disturb other visitors.
This is, of course, really hard to describe to people who are not from Japanese culture, and not entirely predictable. It is never going to stop being difficult to be specific, but fortunately Japan is close enough to “global” standards that common sense does tend to work.
A helpful factor is the principle that everyone should be able to visit a jinja. This means that priests are very reluctant to impose standards that would make it impossible for any group of people to visit, even if some other visitors would be disturbed. Jinja only issue prohibitions when something is both common and a serious problem for a lot of visitors — and this is why I am willing to take Yasukuni’s explanation of the reason behind their ban at face value.
It is worth noting that the idea of not bothering other people is deeply (deeply) engrained in Japanese culture. It is why people line up on escalators, do not use phones on trains, generally avoid eating while walking, and dress in a way that fits in. This is why Japan looks so conformist at first glance — in general, people only do the wild stuff in places where everyone else is comfortable with it as well, or that have become accepted as a place to do that sort of thing.
Dressing in a boring and unremarkable way to visit a jinja is part of the Japanese cultural experience.