A light beam emitted from a light source passes through a spatial light modulation device, at which a plurality of unit elements for respectively modulating incident light beam are two-dimensionally arrayed, and a microlens array, at which a plurality of microlenses corresponding to the unit elements are arrayed, and is focused on an exposure surface. A four-part detector, which is structured by four diodes, is disposed on the exposure surface so as to correspond to four pixels which are present at one corner of the exposure area. Relative mispositioning between the spatial light modulation device and the microlens array generates a difference in respective detection signals of the four diodes. Thus, an offset between the spatial light modulation device and the microlenses can be detected. Positional adjustment of the microlens array is performed on the basis of a detected offset amount.