In a special light mode, first to fourth special images are captured under first to fourth special light being narrow band light. A brightness ratio calculator (60) extracts a blood vessel area containing a blood vessel from each special image. The brightness ratio calculator (60) calculates first to fourth brightness ratios from the special images on every pixel within the blood vessel area. A depth and hemoglobin index calculator (62) calculates the depth of the blood vessel and a hemoglobin index corresponding to the first and second brightness ratios, based on a correlation between the depth of the blood vessel and the hemoglobin index stored in advance. A depth and oxygen saturation calculator (69) calculates an oxygen saturation level corresponding to the third and fourth brightness ratios, based on a correlation between the depth of the blood vessel and the oxygen saturation level stored in advance.
