A substrate processing apparatus includes: a stage configured to place thereon a substrate or at least a portion of a substrate holding member configured to hold the substrate, the substrate having two sides extending in a first direction and two sides extending in a second direction; a transporter configured to transport the substrate to a position facing the stage; first and second sensors, which are fixed relative to the stage and respectively located outside the sides, extending in the second direction, of the substrate at the position facing the stage so as to detect the sides of the substrate, respectively; and third and fourth sensors, which are fixed relative to the stage and respectively located outside the sides, extending in the first direction, of the substrate at the position facing the stage so as to detect the sides of the substrate, respectively.