A detection head includes a plurality of detection array portions that are arranged in a direction along which the head is relatively displaceable. Position signals indicating a position of the detection head are calculated from output signals output from the detection array portions. In order to combine the position signals with one another and determine the position of the detection head with respect to the scale, weighting according to a level of the output signal is performed on the position signal of each of the detection array portions, and the weighted position signals are averaged.