To carry out oxidation of various kinds of material by a method wherein a solvent having a mixture solution of a methyl group or a methylene group, and water is arranged inside an airtight tank, a material to be treated with oxidation is placed on an upper surface of a treating material placed stand at a position not in contact with the liquid surface, and the mixed solution is oxidized with vapour of the solvent and steam by heating.

A material to be treated W is placed on a treating material placed stand 26 of a treating tank T. A mixed solution of a solvent having a methyl group and a methylene group and water is supplied to the treating tank to the degree that the material to be treated with oxidation W is not immersed. Heated steam is sent thereafter, and a vapour of the mixed solution is generated under a specific temperature and pressure state. While the vapour passes through a water layer Wa, the vapour changes to a methyl free group and a methylene free group, which act on the material to be treated with oxidation W such as wood or the like. In the case of wood, a pit part governing going in and out of water in a cell is forcedly dissolved, opened, going out and in of the inside cell water is made easy, immersion of dye and pigment is made easy, and coloring is enabled to be executed. Strain, crack, bend, and damage by worms of the wood are not generated, and highly durable wood is obtained.