Fire officials received an emergency call from a resident at about 5:45 p.m. on Tuesday in Saganoseki, Oita City.
The fire has spread widely, engulfing a residential area and burning surrounding forests. It has still not been brought under control more than 12 hours after the call.
The prefecture says 175 people from 115 households were sheltering at a nearby community hall as of 11:40 p.m. on Tuesday.
Prefectural authorities have decided to put the disaster relief law into effect for the city. This will allow the central government and the prefecture to bear necessary costs, including expenses for maintaining the shelter.
The prefecture has set up a task force and says it will hold a meeting on Wednesday morning to discuss how to proceed.
The residential area involved in the fire is surrounded by mountains about 25 kilometers east of the city center, close to the Saganoseki fishing port.
