A semiconductor device manufacturing method includes an element forming step of forming an clement structure on a front surface of a substrate and forming a back structure on a back surface of the substrate, and a film forming step of performing film forming on a front surface of the element structure while measuring the temperature of the substrate by using a radiation thermometer that applies infrared rays of a wavelength λi to the back structure to obtain an infrared emissivity of the substrate. The back structure has a first layer exposed to the outside and a second layer in contact with the first layer, the refractive index of the second layer being smaller than that of the first layer, and the layer thickness of the first layer is set in a range from (2n−1)λi/8 to (2n+1)λi/8, with n being a positive even number.