To provide a method in which a biomass raw material of vegetable origins of grass, tree, coal or the like is thermally decomposed, CO and H2 are selectively separated from the gas component, the gases of the optimal flow rate are supplied to the polymerization reaction (FT synthesis) by a catalyst, thereby the reaction rate of a catalytic reactor is raised, and the yield of a liquid oil is raised.

The biomass is made to be a raw material, a pyrolysis gas is formed from the raw material by a thermal decomposition furnace 4, CO and H2 are respectively separated from the gas component, each of the gases is separately stored in a H2 storage tank 11 and a CO storage tank 14. The method is characterized by including: a H2 gas flow control valve 12; a CO gas flow control valve 15; a H2 gas flow rate transmitter 13; a CO gas flow rate transmitter 16; and a ratio controller 17 for keeping constant the mutual flow ratio of each of gases, so as to supply the gases by the optimal flow ratio to the polymerization reaction in a catalytic reactor 18.