1. An ultrasound diagnostic apparatus for use with tissue, the ultrasound diagnostic apparatus comprising: a light source configured to irradiate the tissue with light; and a processor programmed to: set light source data indicating a property of the light irradiated to a cross section region of the tissue; set a weight coefficient calculated based on a wavelength and illumination slice of the light source, the weight coefficient reflecting a behavior of the light irradiated in the tissue, the cross-section region data including intensity information on the cross-section region; calculate an illuminance at positions corresponding to coordinates of a plurality of cross-section regions based on the light source data and the weight coefficient, by calculating two-dimensional convolution of the light source data to generate two-dimensional convolution data; calculate illuminance slice data of the plurality of the cross-section regions based on the calculated illuminance by calculating weighted addition of the light source data and the two-dimensional convolution data based on the weight coefficient to create the illuminance slice data; and composite a two-dimensional cross-section image of the tissue from a plurality of the calculated illuminance slice data.