1. A substrate position determining method for performing a positioning of a substrate in a horizontal direction based on output images from a plurality of imaging units disposed along a peripheral portion shape of the substrate, the method comprising: a substrate peripheral portion detecting step for detecting a peripheral portion of the substrate, which is a target for a position detection, based on the output image of each imaging unit; a substrate position deviation correcting step for, when the peripheral portion of the substrate can be detected by at least one imaging unit, acquiring a positional deviation from a preset reference position based on a horizontal position of the substrate obtained from a shape of the detected peripheral portion, and correcting the positional deviation by moving the substrate in a horizontal direction if the deviation exceeds a tolerance range; and a substrate position adjusting step for, when the peripheral portion of the substrate cannot be detected by any imaging unit, obtaining a position adjusting direction of the substrate according to a combination of presence/absence states of the substrate detected based on the output image from each imaging unit, and performing an adjustment of a substrate position by moving the substrate along the position adjusting direction.