A spatial modulator is configured by arranging a plurality of modulating units in an X-axis direction, the modulating units each including at least one light modulating element having a fixed reflecting surface with a constant height from a base surface and a movable reflecting surface with a variable height from the base surface. The spatial modulator spatially modulates a laser beam by the plurality of modulating units. A driver circuit unit applies a voltage corresponding to a driving value to each of the plurality of modulating units, to drive the modulating units individually. A shift amount acquiring portion acquires a driving value at a reference time point and a driving value at an inspecting time point which correspond to a specific light amount value, and acquires a differential value thereof as a shift amount.