1. A diagnosis assisting apparatus, comprising: a tubular tissue extraction means for extracting a tubular tissue region representing a tubular tissue from volume data obtained by imaging and setting a core line to the tubular tissue; a CPR image generation means for generating a CPR image which includes the core line and a longitudinal section of the tubular tissue region; a longitudinal section obtaining means for obtaining the longitudinal section from the CPR image generation means; a pseudo three-dimensional image generation means for generating a pseudo three-dimensional image that includes a mark representing the obtained longitudinal section, as well as the tubular tissue, in the extracted tubular tissue region based on the volume data; and a display control means for causing the pseudo three-dimensional image having the mark to be displayed on a display device, wherein the mark is shaded based on an angle of a normal line at each position of the longitudinal section.