An image acquisition device reciprocates a focal position of an objective lens with respect to a sample in the optical axis direction of the objective lens, while moving a field position of the objective lens with respect to the sample. This makes it possible to acquire contrast information of image data at the field position of the objective lens sequentially as the field position moves with respect to the sample. The image acquisition device acquires the image data by the rolling readout of the image pickup element according to the reciprocation of the focal position of the objective lens.