Supercritical methanol has the potential to protect various monomeric wood-derived products against undesirable degradation to achieve efficient chemical production. In this study, Japanese cedar (Cryptomeria japonica) was treated in supercritical methanol with a semi-flow reactor, adding 10% of water to increase the decomposition rate. During ramp heating at 4°C/min, lignin and hemicelluloses were mainly decomposed around at 270°C, while cellulose was around at 350°C. Coniferyl alcohol and its γ-methyl ether were obtained from lignin in high yield and selectivity.