Employed is an object information acquiring apparatus including an irradiation unit irradiating an object with light a detection unit including a plurality of elements each obtaining an acoustic wave generated from the object and converting the acoustic wave into a detection signal a detection unit moving mechanism moving the detection unit to the object relatively an adding unit selecting, from detection signals converted from acoustic waves obtained by the plurality of elements at respective positions as the detection unit is moved, detection signals converted from acoustic waves obtained from an identical position on the object, and adding together these detection signals to output a resultant summed signal an initial sound pressure computing unit computing an initial sound pressure of a region of interest from the summed signal a light quantity computing unit computing a quantity of light of the region of interest.
