Using a transmittance adjusting device decreasing a transmittance of at least from 460 nm to 500 nm, which is a wavelength range between a point of end of a first optical absorption peak of hemoglobin and a point of start of a second optical absorption peak of hemoglobin, to between 20% and 70% for illuminating light from an illuminating light supply section or reflected light from an object, during observation of the inside of a body cavity, enhanced display of vessels with color reproduction that has no difference from that in observation under visible light, enabling color reproduction similar to that in observation under visible light and further enhanced display of vessels.