This is one of the most interesting sights in Sapporo. WEB site(english) The market has two divisions for Marine Products and Fruit and Vegetables. There is a visit tour started from 5:00am.
Reservation for Visit Tour
They accept an individual visiter and also a group over 10 people available. In case of Group Visit, you have to make a reservation at least 10 days in advance before you visit. In case of individual, you have to make a reservation at least 2 days in advance before you visit. Your reservation is accepted by telephone, FAX, e-mail. E-mail reservation would be better. TEL: 011-611-3176 (office hour: 7:45 – 16:15 Japanese/English, English operator may not be available anytime.）
You are supposed to take a TAXI for access. Public transport is out of service in such a terrible early morning. (*^_^*) You should make a reservation of TAXI in advance. Ask your hotel staff.