1. An ultrasound diagnostic apparatus comprising: a light source information setting unit configured to set light source data indicating a property of a light source irradiated to a cross section region of an object; an optical property setting unit configured to set a weight coefficient indicating an optical property of cross-section region data to the light source, the cross-section region data including intensity information on the cross-section region; an illuminance slice data creation unit configured to calculate illuminance at positions corresponding to coordinates of a plurality of the cross-section regions based on the light source data and the weight coefficient, and create illuminance slice data of the plurality of the cross-section regions based on the calculated illuminance; and a composition unit configured to composite a two-dimensional cross-section image of the object from a plurality of the illuminance slice data.