Abstract: A reactor (3) which accommodates a biomass raw material (B1) and heats and decomposes the biomass raw material (B1) using steam (S); an offgas duct (18) through which offgas (G) generated from the biomass raw material (B1) in the reactor (3) flows; a steam generator (4) which combusts the offgas (G) from the offgas duct (18) to generate the steam (S) and supply the steam (S) to the reactor (3); a supply valve (8) which cuts off the reactor (3) from outside air; an offgas valve (19) which adjusts a flow rate of the offgas (G) in the offgas duct (18); a discharge unit (20) which discharges a processed biomass (B2) produced by heating and decomposing the biomass raw material (B1) in the reactor (3); a discharge valve (21) which opens and closes the discharge unit (20); and a control device (7) which controls the offgas valve (19) so that the offgas (G) is able to be discharged to the offgas duct (18) by depressurizing the reactor (3) at a depressurization speed at which no blasting occurs are provided.