A novel method of biomass processing, in which while carrying out efficient fermentation of biomass, energy can be recovered without the use of plants and in which after the biomass processing the concentration of organic matter in waste liquid can be reduced. There is provided a method of biomass processing, comprising carrying out a hydrogen fermentation of biomass with the use of a hydrogen producing bacterium to thereby recover hydrogen. Further, a fermentation liquid occurring after the hydrogen fermentation is subjected to methane fermentation with the use of a methane bacterium to thereby recover methane. In this instance, at least an organic acid is contained in the fermentation liquid.