After light has been output to a subject to be examined, a photoacoustic wave induced in the subject by the output light is detected. It is assumed that at least one virtual detector element is present outside of a real detector, and dummy data corresponding to the at least one virtual detector element are added to photoacoustic data in which pieces of data of the photoacoustic wave detected by the detector are arranged in accordance with the positions of detector elements. A photoacoustic image is generated by reconstructing the photoacoustic data to which the dummy data have been added by using a Fourier transform method.