Appearance is vitally important in Japan and people are often judged on the way they are dressed.
The business convention of dark suit, shirt and tie is very much the norm in Japanese mainstream business and although other colours are seen more often nowadays than in the past, it is probably safest to maintain a conservative approach.
For women, business dress should be restrained and formal – women do not commonly wear trousers in business in Japan. Accessories should be upmarket but not ostentatious.
Bear in mind that the climate in Japan is very varied through the seasons, so take appropriate wear for the season – overcoats/raincoats may be needed in the winter.