NOVELTY - Producing biogas by solid-state fermentation of a solid biomass material involves: pelletizing the biomass material into pellets; introducing the pellets into an acid fermentation zone; subjecting the biomass material in the pellets to solid-state acid fermentation under facultative anaerobic conditions or under semi-aerobic conditions; densifying the pellets by compression; transferring the densified pellets into a methane fermentation zone; and subjecting the biomass material in the densified pellets to methane fermentation in the methane fermentation zone under anaerobic conditions.    USE - For producing a biogas i.e. methane (claimed).    ADVANTAGE - The process allows to produce the biogas efficiently at a low cost in the two-phase solid-state methane fermentation, in addition to a pelletized solid fuel of good quality obtained from the residue of methane fermentation.    DETAILED DESCRIPTION - Producing biogas by solid-state fermentation of a solid biomass material involves: pelletizing the biomass material into pellets; introducing the pellets into an acid fermentation zone; subjecting the biomass material in the pellets to solid-state acid fermentation under facultative anaerobic conditions or under semi-aerobic conditions kept in an oxygen concentration of 0.01-5%; densifying the pellets by compression to remove the oxygen from among the particles of the biomass material; transferring the densified pellets into a methane fermentation zone; and subjecting the biomass material in the densified pellets to methane fermentation in the methane fermentation zone under anaerobic conditions. An INDEPENDENT CLAIM is included for an apparatus system for producing a biogas by solid-state fermentation of a solid biomass material which comprising an acid fermentation tank (7) having pelletization unit (4) of the solid biomass material and an air inlet (25) with a movable valve (9); and a methane fermentation tank (11) having a pellet compressing unit (12) connected to the acid fermentation tank through a movable valve for separation.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic side view showing the apparatus system.    Pelletization unit (4)    Acid fermentation tank (7)    Movable valve (9)    Methane fermentation tank (11)    Pellet compressing unit (12)    Air inlet. (25)