1. A method of detecting an accessory cancer lesion, comprising: an administration step wherein indocyanine green is administered to a living body, an irradiation step wherein a target organ suspected of having cancer is surgically exposed and irradiated with excitation light of indocyanine green, an imaging step wherein a near-infrared fluorescence intensity distribution image from the excited indocyanine green in the target organ is obtained, and an identification step wherein an area having the near-infrared fluorescence in the intensity distribution image, excluding a main cancer lesion detected in preoperative examination or intraoperative macroscopic observation, is identified as an accessory cancer lesion in the target organ and wherein the imaging step is carried out 1 to 10 days after the administration step.