A photoacoustic wave detector detects a photoacoustic wave generated inside a specimen by light irradiated thereto. A signal processing device: forms first volume data from a first signal, the first signal being the detection signal acquired from the detector or a signal obtained by adjusting an amplitude of the detection signal; forms second volume data from a second signal, the second signal being a signal obtained by changing a phase of the first signal; forms third volume data from the first and second volume data; and generates and outputs image data representing information on an interior of the specimen from the third volume data.