NOVELTY - Production of methane-containing biogas (A) comprises fermentation of biomass (I) at 45-70 (preferably 50-60) degreesC, and optionally inoculating (I) by an inoculum comprising e.g. acetogenic microorganisms; providing anaerobic conditions to the fermentation; before the start of/during the fermentation, inoculating the biogas producing system by a culture of thermophilic, anaerobic, acetogenic, hydrogen-producing bacterium; and collecting/educing (A).    USE - The invention deals about the production of (A), which is a valuable renewable energy carrier; and is useful in heating as well as in electricity generation. (I) is useful as a natural fertilizer.    ADVANTAGE - The method provides (I) in excess quantities. (I) is an excellent natural fertilizer, which is much better than direct application of animal manure. (I) decreases the public health problems associated with waste disposal. The method is simple, economically feasible and technologically stable operation of the simple single reactor systems. The method is cost-effective.    DETAILED DESCRIPTION - Production of methane-containing biogas (A) by fermenting biomass (I) comprising polymer-degrading microorganisms, acetogenic microorganisms and methanogenic microorganisms, under anaerobic conditions, comprises fermentation of (I) by setting its temperature to 45-70degreesC (preferably 50-60degreesC), and optionally inoculating (I) by an inoculum comprising polymer-degrading microorganisms, acetogenic microorganisms and methanogenic microorganisms; providing anaerobic conditions to the fermentation; before the start of/during the fermentation, inoculating the biogas producing system by a culture of thermophilic, anaerobic, acetogenic, hydrogen-producing bacterium; and collecting/educing the produced (A).    An INDEPENDENT CLAIM is also included for a method of increasing biogas production of (A) by fermenting (I) under anaerobic and thermophilic conditions, where the fermentation is carried out by a consortium comprising polymer degrading microorganisms, acetogenic microorganisms and methanogenic microorganisms; before the start of/during the biogas production, a culture of thermophilic, anaerobic, acetogenic, hydrogen-producing microorganism is added one or more times, to the biogas producing consortium.