PURPOSE: To efficiently obtain a raw material for a chemical agent from a ligneous vegetable fiber biomass in a system continuously recycling heat without any environmental pollution by hydrolyzing a lignocellulosic material with a saturated steam and a superheated steam.

CONSTITUTION: A lignocellulosic material such as wood chip is granulated by a hammer mill process 1, fed to a heat exchanger 5 from a high-pressure injector 2, transferred in the heat exchanger 5 at a speed of preferably 30 m/sec and brought into contact with a saturated steam in a blower 4. Then, a high temperature solid flow is fed to a superheated gas flow from a screw 9 through a pressure lowering system 11, hydrolyzed by explosion at the position 14, dried and the treatment is completed after passing through a cyclone 18 and a valve 19.