A semiconductor device manufacturing apparatus and its method, which measures the amount or chemical composition of reaction products adhering to or deposited on the inside of a processing chamber of the semiconductor device manufacturing apparatus, without exposing the chamber to the air. External light such as infrared light is introduced from a light introducing unit into the processing chamber by a light introducing means. A light receiving unit provided outside the processing chamber receives light reflected from a specified location inside the processing chamber or light reflected from an arbitrary location inside the chamber. The received light is then subjected to spectrometry or photometry to judge how badly the chamber is contaminated and to judge the state of the process being carried out.