An image generating apparatus, comprising: a light source adapted to irradiate a pulsed light onto a subject to generate an acoustic wave a probe having a plurality of detection elements adapted to detect the acoustic wave which is generated by irradiation of the pulsed light and propagates from inside the subject and to output a plurality of detection signals corresponding to the plurality of detection elements; a determination unit adapted to determine a first propagation velocity of the acoustic wave; an image generating unit adapted to obtain a first image representing information inside the subject using the first propagation velocity determined by the determination unit and the plurality of detection signals, wherein the determination unit is adapted to: set a target position inside the subject, set a second propagation velocity, determine a detection signal, corresponding to the target position, from among each of the plurality of detection signals on the basis of a distance from each of the plurality of detection elements to the target position and the second propagation velocity, obtain information which represents dispersion of the plurality of detection signals corresponding to the target position, and determine the first propagation velocity based on the information which represents the dispersion of the plurality of detection signals corresponding to the target position, wherein the image generating unit is adapted to obtain a data of a pixel or a voxel included in the first image using a detection signal corresponding to the target position based on a distance from each of the plurality of detection elements to a position corresponding to the pixel or the voxel and the first propagation velocity determined by the determination unit. a light source adapted to irradiate a pulsed light onto a subject to generate an acoustic wave a probe having a plurality of detection elements adapted to detect the acoustic wave which is generated by irradiation of the pulsed light and propagates from inside the subject and to output a plurality of detection signals corresponding to the plurality of detection elements; a determination unit adapted to determine a first propagation velocity of the acoustic wave; an image generating unit adapted to obtain a first image representing information inside the subject using the first propagation velocity determined by the determination unit and the plurality of detection signals, wherein the determination unit is adapted to: set a target position inside the subject, set a second propagation velocity, determine a detection signal, corresponding to the target position, from among each of the plurality of detection signals on the basis of a distance from each of the plurality of detection elements to the target position and the second propagation velocity, obtain information which represents dispersion of the plurality of detection signals corresponding to the target position, and determine the first propagation velocity based on the information which represents the dispersion of the plurality of detection signals corresponding to the target position, wherein the image generating unit is adapted to obtain a data of a pixel or a voxel included in the first image using a detection signal corresponding to the target position based on a distance from each of the plurality of detection elements to a position corresponding to the pixel or the voxel and the first propagation velocity determined by the determination unit. set a target position inside the subject, set a second propagation velocity, determine a detection signal, corresponding to the target position, from among each of the plurality of detection signals on the basis of a distance from each of the plurality of detection elements to the target position and the second propagation velocity, obtain information which represents dispersion of the plurality of detection signals corresponding to the target position, and determine the first propagation velocity based on the information which represents the dispersion of the plurality of detection signals corresponding to the target position, wherein the image generating unit is adapted to obtain a data of a pixel or a voxel included in the first image using a detection signal corresponding to the target position based on a distance from each of the plurality of detection elements to a position corresponding to the pixel or the voxel and the first propagation velocity determined by the determination unit.