A subject is avascularized while changing the avascularization pressure between the avascularized condition and the non-avascularized condition. A receiving circuit receives a detection signal obtained by detecting a photoacoustic wave generated in the subject by emission of measurement light to the subject. Photoacoustic image generating circuitry generates a photoacoustic image based on the detection signal of the photoacoustic wave. The motion of each of a plurality of control points set in each photoacoustic image is detected based on photoacoustic images at a plurality of times. A region of interest is set based on the motion detected at each control point. Blood flow information is generated based on the signal strength of the photoacoustic image in the region of interest.