A methane fermentation method for producing a biogas containing methane from a raw material containing at least one of glycerin and a fatty acid glycerin ester, in which the raw material and an inorganic acid are mixed to form a first oil component and a first glycerin-containing liquid. A first separation step for separating the glycerin, a neutralization step for neutralizing the first glycerin-containing liquid with an alkaline substance, and a second oil and precipitated inorganic salt from the neutralized first glycerin-containing liquid. A methane fermentation method comprising a second separation step of separating to obtain a second glycerin-containing liquid and a fermentation step of performing methane fermentation using the second glycerin-containing liquid. According to the present invention, biogas containing methane can be efficiently produced from glycerin-containing waste and fatty acid glycerin ester-containing waste. In addition, the recovery rate of methane from glycerin-containing waste is dramatically increased as compared with the conventional method.
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