An image generating apparatus of the present invention has a determination unit that sets a target area in a part of an area inside a subject, executes processing to adjust a phase of each detection signal based on a distance from each detection element to a target area and a tentative velocity, and calculate dispersion of intensities of a plurality of detection signals of which phases are adjusted, for a plurality of tentative velocities, and determines a velocity for which dispersion of intensities is minimum, out of the plurality of tentative velocities, as a propagation velocity.