A substrate processing apparatus has a process chamber, a long-shaped window on a wall surface of the process chamber, an irradiator configured to irradiate a first laser beam and a second laser beam to a substrate in the process chamber via the long-shaped window so that incident points of the first and second laser beams are aligned substantially along a long side direction of the window, a detector configured to have a light reception surface receiving the first and second laser beams reflected by the substrate and passing through the window, the detector being configured to detect incident positions of the first and second laser beams on the light reception surface, and a calculator configured to calculate a curvature of the substrate by using a relative position of the first laser beam and the second laser beam which are detected by the detector.