Provided is a substrate processing method comprising: loading a substrate, on which polysilazane is applied, into a substrate process chamber; maintaining an inside of the substrate process chamber, into which the substrate is loaded, in water vapor atmosphere and depressurization atmosphere at a temperature of 400° C.; performing a first heat treatment process on the substrate in a state where the inside of the substrate process chamber is maintained in the water vapor atmosphere and the depressurization atmosphere at the temperature of 400° C.; next, increasing an inner temperature of the substrate process chamber from the temperature of 400° C. in the first heat treatment process to a temperature ranging from 900° C. to 1000° C.; and performing a second heat treatment process on the substrate in a state where the inside of the substrate process chamber is maintained in water vapor atmosphere and depressurization atmosphere at the temperature ranging from 900° C. to 1000° C.