1. An apparatus for generating a photoacoustic image based on an acoustic wave caused by irradiation of light on an object, comprising: a determination unit configured to determine a propagation velocity of the acoustic wave; and an image generation unit configured to generate the photoacoustic image based on the propagation velocity and a signal obtained by detecting the acoustic wave; wherein the determination unit is configured to set, based on an ultrasonic image obtained by transmitting and receiving an ultrasonic wave, a target position, and determine, based on the signal corresponding to the target position, the propagation velocity.