1. A three-dimensional video processing apparatus that performs processing for enlarging a part of 3D video for three-dimensional viewing displayed on a display screen by a predetermined magnification on the display screen, the apparatus comprising: a video obtaining unit configured to obtain the 3D video; a position obtaining unit configured to obtain a position of a first area which is an area included in the part of the 3D video; a disparity obtaining unit configured to obtain disparity amounts of the 3D video per processing unit obtained by dividing the 3D video by a predetermined number of pixels; a determination unit configured to determine a size, on the display screen, of an enlargement area based on a size of the display screen and the predetermined magnification, the enlargement area being included in the part of the 3D video; and an area selection unit configured to select, as the enlargement area, at least one of a plurality of second areas that includes processing units each having a disparity amount within a predetermined range, the second areas being included in the part of the 3D video, and each including the first area and having a size determined by the determination unit.