1. A stereoscopic image processing apparatus comprising: an input interface configured to acquire an input stereoscopic image including an input left eye image and an input right eye image, the input left eye image and the input right eye image having first parallax in a first parallax direction and including a subject; and an output interface configured to output an output stereoscopic image including an output left eye image and an output right eye image, the output left eye image being based on the input left eye image, the output right eye image being based on the input right eye image, the output left eye image and the output right eye image having second parallax in a second parallax direction and including the subject, wherein when the subject in the input stereoscopic image has a first width in the first parallax direction, each of first edge areas of the subject is given a first depth in the second parallax in the output stereoscopic image, wherein the first edge areas of the subject in the output stereoscopic image are aligned in the second parallax direction, and when the subject in the input stereoscopic image has a second width larger than the first width in the first parallax direction, each of second edge areas of the subject is given a second depth in the second parallax in the output stereoscopic image, the second depth being deeper than the first depth, wherein the second edge areas of the subject in the output stereoscopic image are aligned in the second parallax direction.