A treatment method for biomass which is a pretreatment method including continuously performing hydrothermal treatment using a screw extruder to produce a biomass composition for saccharification from cellulose-containing biomass serving as a raw material with a small amount of electricity. The treatment method sequentially includes: pulverizing, in a pulverization section of the screw extruder, the cellulose-containing biomass so as to have a maximum grain size of 1,000 μm or less and adjusting a water content ratio thereof to from 30% to 80%; performing, in a heating section of the screw extruder, hydrothermal treatment at a temperature of from 205° C. to 250° C. for 0.1 minute to 10 minutes while performing kneading pulverization having a grinding effect; and cooling, in a cooling section of the screw extruder downstream of the heating section, a treated product to 100° C. or less to recover the treated product.