NOVELTY - Method involves feeding liquefied organic wastes into methane tank; providing sequential anaerobic fermentation of organic wastes in methane tank; heating and mixing fermented mass; discharging fermented mass and withdrawing biogas and holding dense and settling difficult-for-fermentation substances in outer chamber for time sufficient for complete hydrolytic splitting in acid medium.    USE - Agriculture, in particular, method and equipment used in animal and poultry farms for producing of fuel gas from manure, bird dung, plant remains, or for manufacture of high-quality organic fertilizers.    ADVANTAGE - Increased efficiency by providing complete splitting of dense settling difficult-for-fermentation liquefied organic substances, increased amounts and improved caloricity of biogas produced.7 cl, 2 dwg    DETAILED DESCRIPTION - Method involves feeding liquefied organic wastes into outer chamber of methane tank; providing sequential anaerobic fermentation of organic wastes in outer and inner chambers of methane tank; heating and mixing fermented mass; discharging fermented mass from methane tank and withdrawing biogas; holding dense and settling difficult-for-fermentation substances, introduced into outer chamber of methane tank together with liquefied organic wastes, in outer chamber for time sufficient for complete hydrolytic splitting thereof in acid medium produced during first fermentation phase; directing decomposition products thereof into alkaline medium of second fermentation phase in inner chamber. Apparatus has methane tank with outer chamber equipped with annular low wall made in the form of truncated cone defining tray with horizontal upper part and inclined base. Annular low wall is adapted for holding of dense settling difficult-for-fermentation organic substances in outer chamber. Such construction of outer chamber provides for convenient discharge of settled substances from jet nozzles of annular pipe above annular low wall defining tray. Washing out pipe is connected to pump for mixing of mass under fermentation process.