To enable improved image photographing while securing sufficient blood supply for an organ region supplied with blood via a blood vessel to be tested.

An image photographing unit for radiating light and receiving light reflected by walls of blood vessels is incorporated in a catheter, and the catheter is used for a method for photographing two-dimensional images of the inside of the blood vessels in blood streams by optical coherence tomography. For photographing the images via the catheter, a liquid containing a medium having a refractive index higher than that of blood plasma and/or cytoplasm is injected in the region of the blood vessels to be photographed, and the liquid is a mixed liquid made of a self blood and the medium.