A method for continuous saccharification of plant raw material by feeding the raw material as such or prehydrolyzed and a dilute sulphuric acid solution into a flow reactor in order to hydrolyze the raw material under pressure and at a high temperature, discharging dry solids and liquid from the reactor by expansion and separating liquid and solids, wherein the raw material and sulfuric acid solution are led through the reactor at equal rate, which is the same as required by the more easily hydrolyzable particles of the raw material, the solids and liquid together are discharged into the same blow tank and at least part of the separated coarse solid is returned into the reactor.