1. An image generating apparatus, comprising: a plurality of detection elements which detect an acoustic wave propagating from inside a subject and output detection signals; a determination unit that determines a propagation velocity of the acoustic wave inside the subject; generating unit that generates data representing information inside the subject using the propagation velocity determined by said determination unit and a plurality of detection signals obtained from said plurality of detection elements, wherein said determination unit sets a target position reflecting subject information, out of data generated based on a predetermined propagation velocity and the plurality of detection signals, sets a tentative propagation velocity, determines a plurality of signals corresponding to the target position, out of the plurality of detection signals based on a distance from each of said detection elements to the target position and the tentative propagation velocity, and determines the propagation velocity based on the plurality of signals corresponding to the target position.