1. A stereoscopic viewing medical image generation apparatus, comprising: a stereoscopic viewing image generation unit configured for, using a three-dimensional medical image representing a subject as input, generating a stereoscopic viewing image for stereoscopic output of the subject based on a given image generation condition, a given image generation condition of the image generation unit including a number of parallax images of the stereoscopic viewing image, viewpoint positions of the stereoscopic viewing image, visual line directions of the stereoscopic viewing image, projecting with parallel projection or central projection, and image generation; a non-stereoscopic viewing image generation unit configured for generating a non-stereoscopic viewing image for non-stereoscopic output equivalent to the stereoscopic output based on the three-dimensional medical image and the image generation condition of the stereoscopic viewing image; a selective output unit configured for selectively outputting the stereoscopic viewing image and the non-stereoscopic viewing image based on a predetermined output image selection condition; and a cross-sectional image generation unit configured for generating a cross-sectional image based on three-dimensional medical image and position information, wherein an output destination device of the selective output unit is capable of selectively presenting a stereoscopic display based on the stereoscopic viewing image and a non-stereoscopic display based on the non-stereoscopic viewing image, the output image selection condition is a condition that, when an operation of an input device involving specification of a desired position in the stereoscopic viewing image stereoscopically displayed on the display device is detected, causes the non-stereoscopic viewing image to be outputted, the operation is an operation for specifying position that is a part of the three-dimensional medical image to stereoscopically display or an operation for specifying position to generate cross-sectional image, and after receiving the desired position specified on the non-stereoscopic viewing image after the operation, the selective output unit generates a stereoscopic viewing image corresponding to the specified position by the stereoscopic viewing image generation unit or generates a non-stereoscopic viewing image corresponding to the specified position by the non-stereoscopic viewing image generation unit based on the operation.