I've been invited to a workshop in Japan, and I haven't been to an academic event there before. In my field in the UK, it would be normal to attend an event in "smart casual" wear, eg a polo top or open necked shirt. Should I expect this convention to be different in Japan?
From going to Japan for math, engineering, and Earth sciences conferences a number of times, expect that every Japanese participant is wearing a dark suit and a more or less colorless tie. (There are very few women in science in Japan, so I don't have a lot of experience in this regard.) This includes student workers -- also in suits. You -- and all of the other foreigners who didn't get this memo :-) -- will stick out of the crowd if you wear anything that has color, or just a light blazer. Depending on how many other foreigners are there, you will be ok, though, with an open neck button down shirt, dark pants, and a blazer.
You will find that you can really stick out of the crowd on the streets if you wear an outfit that is colorful. This may just be a pair of light bluejeans and a dark jacket (nobody seems to be wearing light blue pants). You will really really stick out of the crowd if you walk from the train station to the hotel after a long flight with an orange or blue wind jacket and a reddish suitcase. I speak from experience :-)
I cannot hide there anyway, being tall and with medium brown hair. But whenever I go out there, I now usually wear gray or black slacks (or black jeans), and a black jacket. I can't get myself to wear a white shirt and dark tie, so I might have an off white or light blue button down.
Whatever you do, though, you will always be treated nicely. It's just a question of how "different" you want to be.
I did my PhD in Japan, in Sociology, and my experience at Japanese national conferences and workshops was just like the other answers described: black suits and white shirts everywhere. The only ones who deviated from the norm were senior scholars.
However, after moving out of Japan, I was back for an international conference there and was surprised to see the same scholars who wore the dark suit uniform for national events wearing much more casual attire. My point is that, besides variations by field, Japanese scholars are aware that things are more casual in the West and are willing to accommodate these differences.
Something that hasn't really been highlighted is that Japanese will be very accommodating regarding different outfits (worn by foreigners). You might feel out-of-place, but looking different (you're not going to wear ripped low-hanging jeans, right?) from the standard isn't necessarily perceived as being under-dressed. Social rules usually apply only in Japan and only for Japanese, or rather for inter-Japanese relationships.
I don't know how "international" this conference will be. Often researchers with a lot of international experience (that is, papers with foreign collaborators, frequent visits to international conferences outside of Japan, etc.) will dress more like foreigners, i.e. a more leisurely outfit. They might wear a suit, though, if their head of school is going to attend.
Conferences in fields (or sub-fields) that address a national crowd will likely follow the mentioned Japanese standard (black suit, white shirt) very closely, but maybe only because it is a social event in Japan, for Japanese and should thus follow the Japanese norm.
Disclaimer: my field is Pure Mathematics.
I live in Japan and work in academia there, and I always wear the same respectable casual attire that I would wear to work in the UK, as do all of my foreign colleagues and most of my Japanese ones. Some Japanese people dress more smartly, but nobody seems to mind the mix. It's been pretty much the same at the international conferences I've been to in Japan as well. My field is "pure science" if that makes a difference.
I was at a major (for my field) academic conference in Japan last summer and I can't say I noticed the people at the conference wearing anything different to what they wear at the conferences I've been to in Europe and North America. If the majority of participants at the event you're going to are not Japanese, I would imagine you'd find exactly the same thing.
This depends on which field you're in. According to my own experience, typically, in engineering workshops people tend to dress semiformally, where as in pure science people usually dress casually. The idea is, I suppose, is that in the former you need to look like a businessperson. You might be able to extrapolate this to your own situation. Good luck!