A substrate processing apparatus includes a placing table, having a first placing surface on which a substrate is placed and a rear surface opposite to the first placing surface, provided with a first window which allows the first placing surface and the rear surface to communicate with each other and which is configured to transmit light; a first adjusting device configured to hold a first light irradiation unit configured to irradiate light toward the first window and configured to adjust an irradiation position of the light on the rear surface; and a first reflection member, having retroreflection property, disposed at the rear surface of the placing table to enclose the first window, and configured to reflect a part of the light and return reflection light indicating a deviation between the irradiation position of the light and a position of the first window to the first light irradiation unit.