This invention provides a steam oxidation method of a matter to be oxidized with proper controllability and reproducibility. It is provided a steam oxidation method, where a semiconductor substrate (a matter to be oxidized) is housed in a steam oxidation reactor and is subjected to: a first step of supplying N2 gas to the reactor housing the semiconductor substrate and substituting the inside of the reactor with N2 gas; a second step of stopping supply of the N2 gas and supplying a steam-accompanied N2 gas, in which the N2 gas is accompanied with steam, to the reactor; a third step of increasing a temperature of the semiconductor substrate to 450° C. (a steam oxidation temperature) while supplying the steam-accompanied N2 gas; and a fourth step of holding the semiconductor substrate for a predetermined time at 450° C.