A substrate processing apparatus includes a stage, a light source, an optical assembly, a light receiver, and controller circuitry. The stage includes a first placing surface on which a substrate is to be placed, and a second placing surface that surrounds the first placing surface and on which a focus ring is to be placed. The optical assembly focuses light from the light source on a lower surface position, which is a position of a lower surface of the focus ring placed on the second placing surface. The light receiver receives light from the lower surface position. The controller circuitry detects at least one of a presence and an absence of the focus ring on the second placing surface, based on light received by the light receiver.