A methane fermentation method having the steps of finely pulverizing an organic substance adjusted such that a moisture content of the organic substance is equal to or less than 10%, in a wet bead mill; supplying the organic substance which is finely pulverized in the pulverization step to a methane fermentation chamber and decomposing the organic substance with an anaerobic microorganism to cause methane fermentation so as to generate methane gas; separating, with a UF membrane, a digestive fluid after the methane fermentation in the methane fermentation step into a concentrated liquid and a permeation liquid; returning the concentrated liquid separated in the separation step to the methane fermentation chamber; and agitating the contents of the methane fermentation chamber with a water flow produced by the return of the concentrated liquid in the return step.