NOVELTY - The installation comprises: a fibrous biomass dilution tank (10) for forming a mixture containing the fiber suspended in a liquid and for hydrolysis of dry matter in the mixture; a roof movably mounted on the dilution tank for maintaining the dry matter fiber immersed in the liquid; an actuator for actuating the roof; a liquid-phase milling device to crush the dry matter; a phase separator (12) for separating the dry matter from the liquid mixture; and a dryness adjusting unit. The actuator forms a unit for adjusting the roof relative to the dilution tank.    USE - The installation is useful for preparing biogas substrate from solid fibrous biomass and in a biogas production system (all claimed).    ADVANTAGE - The installation can be operated for a long time without degrading its performance, and enables effective and stable production of biogas with high yield.    DETAILED DESCRIPTION - The installation comprises: a fibrous biomass dilution tank (10) for forming a mixture containing the fiber suspended in a liquid and for hydrolysis of dry matter in the mixture; a roof movably mounted on the dilution tank for maintaining the dry matter fiber immersed in the liquid; an actuator for actuating the roof; a liquid-phase milling device to crush the dry matter; a phase separator (12) for separating the dry matter from the liquid mixture; and a dryness adjusting unit. The actuator forms a unit for adjusting the roof relative to the dilution tank. The roof is perforated by an orifice. The liquid-phase milling device comprises a substrate. The milling device is arranged: within the phase separator; or between the dilution tank and the dryness adjusting unit.    INDEPENDENT CLAIMS are included for:    (1) a system for producing biogas; and    (2) a method for preparing a biogas substrate from solid fibrous biomass.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a biogas production system.    Fibrous biomass dilution tank (10)    Phase separator (12)    Horizontal agitator (20)    Vertical agitator (22)    Downstream crushing device. (24)