Subject-distance information is acquired by a light-weight, small-size arrangement without a significant change in the structure of a conventional image processing apparatus. The apparatus acquires diffraction images obtained by sensing the image of a subject, using an image sensing unit, obtained via a diffraction grating and imaging optical system, detects a real image from luminance gradients of the diffraction images, calculates the distance between the detected real image and a virtual image corresponding to this real image in the diffraction images, and calculates the depth distance between the subject and the diffraction grating using the distance calculated by first calculating unit.