1. A three-dimensional image processing apparatus for processing two-dimensional viewpoint images corresponding respectively to right and left eyes of one or more users to display a three-dimensional image perceived by the one or more users, the apparatus comprising: a storage unit configured to store a parallax amount range; and a control unit configured to adjust parallax of the two-dimensional viewpoint images in accordance with the parallax amount range, wherein the control unit is configured to slide one of the two-dimensional viewpoint images in a horizontal direction relative to another two-dimensional viewpoint image to: adjust a first parallax amount to be within the parallax amount range when the first parallax amount is outside the parallax amount range, the first parallax amount allowing a three-dimensional image to be closer to the one or more users, adjust a second parallax amount to be within the parallax amount range when the second parallax amount is outside the parallax amount range, the second parallax amount allowing a three-dimensional image to be distant from the one or more users, and adjust the first parallax amount to be within the parallax amount range when the first and second parallax amounts are outside the parallax amount range.