An image processing system comprising: an acquisition unit configured to acquire a rendering condition used for generating a parallax image group that is a predetermined parallax number of parallax images from volume data that is three-dimensional medical image data; a determination unit configured: to set corresponding information based on at least a parallactic angle between parallax images constituting the parallax image group included in the rendering condition and a display size of the parallax image group displayed on a stereoscopic display device , the corresponding information for causing a space coordinate of a stereoscopic image viewed stereoscopically by referring to the stereoscopic display device that displays the parallax image group to correspond to a space coordinate of a captured site in the volume data; and to determine a scale for converting a length in a perpendicular direction with respect to a display surface of the stereoscopic display device in a space of the stereoscopic image into a length in a space of the captured site based on the corresponding information, the perpendicular direction being both a protruding direction with respect to the display surface and a depth direction with respect to the display surface and the scale is in both the protruding direction and the depth direction, wherein the determining of the scale is modified in accordance with a protruding limit value and a depth limit value, which both depend on specifications of the stereoscopic display device and which express a limitation of a stereoscopic effect sensed by an observer; and an output unit configured to perform output control such that the scale is displayed on the stereoscopic display device in a manner superimposed on the stereoscopic image based on the parallax image group. an acquisition unit configured to acquire a rendering condition used for generating a parallax image group that is a predetermined parallax number of parallax images from volume data that is three-dimensional medical image data; a determination unit configured: to set corresponding information based on at least a parallactic angle between parallax images constituting the parallax image group included in the rendering condition and a display size of the parallax image group displayed on a stereoscopic display device , the corresponding information for causing a space coordinate of a stereoscopic image viewed stereoscopically by referring to the stereoscopic display device that displays the parallax image group to correspond to a space coordinate of a captured site in the volume data; and to determine a scale for converting a length in a perpendicular direction with respect to a display surface of the stereoscopic display device in a space of the stereoscopic image into a length in a space of the captured site based on the corresponding information, the perpendicular direction being both a protruding direction with respect to the display surface and a depth direction with respect to the display surface and the scale is in both the protruding direction and the depth direction, wherein the determining of the scale is modified in accordance with a protruding limit value and a depth limit value, which both depend on specifications of the stereoscopic display device and which express a limitation of a stereoscopic effect sensed by an observer; and an output unit configured to perform output control such that the scale is displayed on the stereoscopic display device in a manner superimposed on the stereoscopic image based on the parallax image group. <EOS>