One sensor constituted of a light emission element and a light-receiving element is provided in a path through which a wafer is transferred. The sensor is positioned so that the wafer passes through an area between the light emission element and the light-receiving element. Coordinates of the center of the wafer are calculated based on encoder values obtained when the wafer starts passing through the sensor and when the wafer completes passing through the sensor, position data of wafer transfer means corresponding to the encoder value, and the diameter of the wafer; and thereby the amount of positional deviation of the center of the wafer from a reference position is calculated.