To more effectively continue methane fermentation even when the methane fermentation is performed without performing water addition to an organic substance in fact such as the so-called dry methane fermentation.

The fermentation gas production apparatus provided with a methane fermentation tank in which the organic substance is introduced from an upper part and a residue of the organic substance after fermentation is discharged from a lower part is provided with a rotation shaft extending in a vertical direction in the methane fermentation tank; a stirring member extending in a radial direction of the rotary shaft, provided on the rotary shaft at a multi-stage while leaving in a vertical direction and stirring the organic substance in the methane fermentation tank by rotation of the rotary shaft; and a jet part provided on at least one stirring member and jetting a purification gas performed with a purification treatment, i.e., a part of the gas generated in the methane fermentation tank.