To ensure a dwell time necessary and sufficient for fermentation by allowing an organic matter-containing liquid such as excretions or the like of livestock to reciprocally flow in a fermentation tank when the organic matter-containing liquid is fermented in the fermentation tank to form methane gas.

The organic matter-containing liquid from a receiving tank 22 is fermented while allowed to successively overflow a plurality of fermentation tanks 21 and the formed methane gas is recovered in a gas holder 32. The liquid temperatures of the respective fermentation tanks 21 are set to values becoming high stepwise toward the downstream side and the organic matter-containing liquid overflowing from the upstream side gets into the bottom part of each fermentation tank 21 to gradually rise by the liquid density difference caused by the liquid temperatures becoming high stepwise.