An image data acquisition apparatus comprising: a reception unit configured to output signals by receiving acoustic waves generated from an object that has been irradiated by light, a first acquisition unit configured to acquire a first plurality of image data sets of the object corresponding to a plurality of signals obtained by receiving, using the reception unit, acoustic waves generated by a plurality of irradiations of the object by light; a second acquisition unit configured to acquire first composited image data by selecting a first two or more image data sets of the first plurality of image data sets corresponding to two or more irradiations by light performed during a first period; characterised in that the apparatus further comprises: a third acquisition unit configured to acquire second composited image data, using a second two or more image data sets of the first plurality of image data sets corresponding to two or more irradiations by light performed during a second period that is different from the first period, the second two or more image data sets being a combination different from the first two or more image data sets; a fourth acquisition unit configured to acquire information relating to positional displacement between the first composited image data and the second composited image data, using the first composited image data and the second composited image data; and a fifth acquisition unit configured to acquire information relating to positional displacement of the first plurality of image data sets, using information relating to the positional displacement between the first composited image data and the second composited image data. a reception unit configured to output signals by receiving acoustic waves generated from an object that has been irradiated by light, a first acquisition unit configured to acquire a first plurality of image data sets of the object corresponding to a plurality of signals obtained by receiving, using the reception unit, acoustic waves generated by a plurality of irradiations of the object by light; a second acquisition unit configured to acquire first composited image data by selecting a first two or more image data sets of the first plurality of image data sets corresponding to two or more irradiations by light performed during a first period; characterised in that the apparatus further comprises: a third acquisition unit configured to acquire second composited image data, using a second two or more image data sets of the first plurality of image data sets corresponding to two or more irradiations by light performed during a second period that is different from the first period, the second two or more image data sets being a combination different from the first two or more image data sets; a fourth acquisition unit configured to acquire information relating to positional displacement between the first composited image data and the second composited image data, using the first composited image data and the second composited image data; and a fifth acquisition unit configured to acquire information relating to positional displacement of the first plurality of image data sets, using information relating to the positional displacement between the first composited image data and the second composited image data. a third acquisition unit configured to acquire second composited image data, using a second two or more image data sets of the first plurality of image data sets corresponding to two or more irradiations by light performed during a second period that is different from the first period, the second two or more image data sets being a combination different from the first two or more image data sets; a fourth acquisition unit configured to acquire information relating to positional displacement between the first composited image data and the second composited image data, using the first composited image data and the second composited image data; and a fifth acquisition unit configured to acquire information relating to positional displacement of the first plurality of image data sets, using information relating to the positional displacement between the first composited image data and the second composited image data.