A heat treatment method which can prevent heat deformation of a substrate caused during a heat treatment process on the substrate with a thin film formed on its surface is provided. The heat treatment method in accordance with the present invention includes (a) stacking a second substrate 10b on a first substrate 10a; and (b) stacking a weight 20 on the second substrate 10b, wherein the first substrate 10a and the second substrate 10b are stacked, with thin films 12 of the substrates 10a and 10b being in contact with each other. In accordance with the present invention, deformation of the substrate can be prevented by stacking the substrates, with thin films formed on the substrates being in contact with each other, and placing a weight on the stacked substrates during the heat treatment process.