In order to convert woody waste into methane gas efficiently, the pretreatment effect of steam explosion and the additive effect of soybean curd residue, okara, were clarified. 180 mL of methane gas was obtained from 1 g of steam-exploded Japanese cedar chips at a steam pressure of 4.51 MPa and a steaming time of 5 min while no methane gas was produced from untreated chips. The addition of okara into the chips was attempted and the optimal condition, i.e. C/N of 18, increased the methane gas produced to 315 mL.