Illumination light projected into a body cavity includes first to third narrowband rays of different wavelength ranges, at least one of these narrowband rays has a central wavelength of not more than 450 nm. Under these narrowband rays, first to third narrowband image signals are respectively obtained through an endoscope. Based on the first to third narrowband image signals, vascular areas containing blood vessels are determined, and a first luminance ratio between the first and third narrowband signals and a second luminance ratio between the second and third narrowband signals are calculated at every pixel of the vascular areas. From the calculated first and second luminance ratios, information about both the depth and oxygen saturation of the blood vessels is acquired with reference to correlation data that correlates the first and second luminance ratios to the vessel depth and the oxygen saturation.