Abstract: An apparatus for determining concentrations of hemoglobins additionally uses a light source 3 which emits light of a third wavelength in an orangy red wavelength region of 590 to 660 nm. The apparatus includes light receiving means 6 for receiving lights that are emitted by the light sources and transmitted through or reflected by a living tissue, attenuation ratio processing means 15 for processing attenuation ratios &PHgr; on the wavelengths based on variations of signals associated with the wavelengths output from the light receiving means, which variations are caused by a pulsation of blood, and concentration ratio processing means 16 for processing concentration ratios of at least oxyhemoglobin, deoxyhemoglobin and carboxyhemoglobin based on the output signals from the attenuation ratio processing means. The apparatus thus constructed can properly measure carboxyhemoglobin COHb, and present its concentration display and an alarm display in a simple manner, which is clinically effective.