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.
