A fixed-point detector is provided. The fixed-point detector includes a plurality of fixed-point detecting patterns, a fixed-point detecting light source, and a plurality of photoelectric conversion elements. The plurality of fixed-point detecting patterns each have a pair of diffraction gratings for diffracting incident light in different directions. The fixed-point detecting light source irradiates the pair of diffraction gratings with light while moving in the measurement-axis direction with respect to the plurality of fixed-point detecting patterns. The plurality of photoelectric conversion elements move together with the fixed-point detecting light source, while receiving light beams diffracted by the respective diffraction gratings of the plurality of fixed-point detecting patterns and converting the diffracted light beams into electric signals.