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; 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; and an evaluation value calculation unit configured to calculate an evaluation value of each of the processing units, wherein the evaluation value is a sum of a first evaluation value and a second evaluation value, the first evaluation value decreasing as an absolute value of the disparity amount of the processing unit is less, and the second evaluation value decreasing as a position of the processing unit on the display screen is closer to the position of the first area, and the area selection unit is configured to select, as the enlargement area, at least one of the second areas that includes processing units having a smallest sum of the evaluation values.