A biomass treatment device and a biomass treatment method, in which contamination is prevented and a final product can be highly concentrated. The biomass treatment device includes a pretreatment device for pretreating a biomass, a slurrying device for slurrying the pretreated biomass, and a saccharification device for saccharifying the slurried biomass with a saccharifying enzyme. A return line for returning a part of an internal fluid in the saccharification device to the slurrying device is provided, and the pretreated biomass from the pretreatment device and the internal fluid returned through the return line are mixed in the slurrying device so as to generate biomass slurry.