PURPOSE: To utilize the fermentation heat, generated naturally from the rice bran, effectively by a simple equipment without consuming fuel by a method wherein a fermentation tank is kept warm and the fermentation heat is accumulated once in water in the heat accumulating tank, which has the largest specific heat, to effect heat exchange efficiently.

CONSTITUTION: The fermentation tank 1 is provided with a cylindrical heat accumulating tank 5, containing water, at the central part thereof while the flange 5a projected at the upper part of the heat accumulating tank 5, is connected to an upper lid 4 and the tank 5 is provided in a condition that it is hung with some clearance between a bottom wall 3. The heat accumulating tank 5 is covered by a lid 17 so as not to release heat accumulated in the tank. Spiral heat exchanging tubes 6 are arranged in the heat accumulating tank 5 so as to increase the area, contacting with water B, as much as possible. A mixing device 7 consists of a plurality of mixing blades 20, a mixing rod 21, extending and securing the mixing blades 20, a mixing shaft 22 connected with the mixing rod 21, and a bearing 23 secured to the bottom wall 3 and supporting rotatably the mixing shaft 22.