To provide a methane fermentation tank capable of suppressing the effect of atmospheric temperature, capable of reducing the diffused quantity of heat, capable of reducing the area of the surface of free water and capable of easily crushing formed scum.

A cylindrical tank body 2 partially embedded in the ground is arranged so that the axis (a) of the tank body 2 is inclined by a predetermined angle with respect to ground surface and the corner part 2a on one side of the tank body 2 is exposed to ground surface and the periphery of the tank body 2 is filled with a heat insulating agent to form a heat insulating layer 5.