On the +X and −X sides of a projection unit, a plurality of Y heads are arranged in parallel to the X-axis by a distance half or less than half the effective width of the scale, so that two heads each constantly form a pair and face a pair of Y scales. Similarly, on the +Y and −Y sides of the projection unit, a plurality of X heads are arranged in parallel to the Y-axis by the distance, so that two heads each constantly form a pair and face a pair of X scales. Of the pair of heads consisting of two heads which simultaneously face the scaler measurement values of a priority head is used, and when abnormality occurs in the measurement values of the priority head due to dust and the like adhering on the scale surface, measurement values of the other head is used. By using the two pairs of Y heads and the pair of X heads, a position of a stage within a two-dimensional plane is measured in a stable manner and with high precision.