An optical sensor for detecting the housing state such as the thickness of a substrate in the present invention is provided at supporting arms. The supporting arms are attached to a supporting shaft. The supporting arms are in a vertical state in a state before detection of the substrate, but when detecting, the supporting shaft rotates to bring the supporting arms into a horizontal state so that the optical sensor enters a substrate housing body and is set at a predetermined detection position. Accordingly, a space for moving the optical sensor in the horizontal direction becomes unnecessary to reduce the space required for the detecting operation and the like, making it possible to reduce the size of a substrate processing apparatus in which the detecting apparatus is incorporated.