A subject information acquiring device has: an acoustic detector which receives an acoustic wave propagated in a subject and converts into an electrical signal and a data processing device which generates a subject information distribution using the electrical signal, and the data processing device has a matching processing unit which acquires a similarity distribution by calculating a similarity between: template data indicating a relationship between a real image and an artifact and the subject information distribution used as a matching information distribution, so that it is possible to provide a photoacoustic image forming diagnosing device which can distinguish between an image and a background even when the contrast is poor.
