A plant biomass pretreatment method which allows prompt pretreatment of plant biomass with simple equipment is provided. The method includes continuously performing in sequence, inside an extruder, pretreatment steps of coarsely crushing the plant biomass to a predefined size or smaller, adding a decomposing agent(s) to the coarsely crushed plant biomass, applying a hot compressed water treatment(s) to the plant biomass with the decomposing agent added thereto, and performing saccharification preparation for mixing the plant biomass with the hot compressed water treatment applied thereto with an enzyme(s) for saccharifying the plant biomass.