A photoacoustic signal processing device comprising: acoustic wave detection means for detecting a photoacoustic wave emitted from an inserted object inserted into a subject, the inserted object having a light guide member configured to guide light from a light source and a photoacoustic wave generation unit configured to absorb light emitted from the light guide member to generate the photoacoustic wave; photoacoustic image generation means for generating a photoacoustic image based on a detection signal of the photoacoustic wave; mode setting means for setting a peak search mode for detecting the photoacoustic wave in a plurality of postures while the posture of the acoustic wave detection means is changed and a normal mode for displaying the photoacoustic image; peak specification means for specifying, based on a plurality of photoacoustic images according to the detection signals of the photoacoustic waves detected in the plurality of postures, a photoacoustic image with the strongest detection signal of the detected photoacoustic wave among the plurality of photoacoustic images in the peak search mode; posture determination means for determining whether or not the posture of the acoustic wave detection means at the time of detecting photoacoustic waves of a generation source of the photoacoustic image matches the posture of the acoustic wave detection means at the time of detecting photoacoustic waves of a generation source of the photoacoustic image specified by the peak specification means in the normal mode; and display control means for further displaying the photoacoustic image on a display screen, and in a case where the posture determination means determines that the postures match each other, displaying a report indicating the postures matching each other on the display screen in the normal mode. acoustic wave detection means for detecting a photoacoustic wave emitted from an inserted object inserted into a subject, the inserted object having a light guide member configured to guide light from a light source and a photoacoustic wave generation unit configured to absorb light emitted from the light guide member to generate the photoacoustic wave; photoacoustic image generation means for generating a photoacoustic image based on a detection signal of the photoacoustic wave; mode setting means for setting a peak search mode for detecting the photoacoustic wave in a plurality of postures while the posture of the acoustic wave detection means is changed and a normal mode for displaying the photoacoustic image; peak specification means for specifying, based on a plurality of photoacoustic images according to the detection signals of the photoacoustic waves detected in the plurality of postures, a photoacoustic image with the strongest detection signal of the detected photoacoustic wave among the plurality of photoacoustic images in the peak search mode; posture determination means for determining whether or not the posture of the acoustic wave detection means at the time of detecting photoacoustic waves of a generation source of the photoacoustic image matches the posture of the acoustic wave detection means at the time of detecting photoacoustic waves of a generation source of the photoacoustic image specified by the peak specification means in the normal mode; and display control means for further displaying the photoacoustic image on a display screen, and in a case where the posture determination means determines that the postures match each other, displaying a report indicating the postures matching each other on the display screen in the normal mode.