A semiconductor device manufacturing apparatus has a first space and a second space in a process chamber in which a semiconductor wafer is accommodated, the first and second spaces being separated by the semiconductor wafer. A process gas port opens into the first space adjacent to the obverse surface of the semiconductor wafer, and an infrared light transmission window is formed in a wall of the chamber at the second space facing the reverse surface of the semiconductor wafer. No layers are deposited on the reverse surface of the semiconductor wafer and the infrared light transmission window so that the emissivity at the reverse surface of the semiconductor wafer is not changed during layer deposition. The temperature during processing can therefore be monitored accurately with a pyrometer, and a reduction in the transmissivity of the window is prevented.