An image of a tissue site including a blood vessel is captured while broadband light and narrow band light are emitted to a body cavity. Thereby, broadband image data corresponding to the broadband light and narrow band image data corresponding to the narrow band light are obtained. Picture elements in the same positions are identified between the broadband image and the narrow band image to obtain a brightness ratio LM therebetween. Based on depth correlation information between the brightness ratio and blood vessel depth, a blood vessel depth D corresponding to the brightness ratio is obtained to determine whether each picture element includes a blood vessel and whether the blood vessel depth D is at the surface. Based on the determination, a surface blood vessel region (71) is extracted. A broadband image (80) having the surface blood vessel region with the reduced contrast is generated.
