In a pulse oximeter for obtaining an oxygen saturation in a blood, a light emitter irradiates a living tissue with light beams having five different wavelengths. A light receiver receives respective light beams reflected from or transmitted through the living tissue, and converts the received light beams to electric signals. A first calculator calculates five attenuation changes of the living tissue based on fluctuations of the respective electric signals. A second calculator calculates at least four attenuation change ratios from the five attenuation changes. Each of the attenuation change ratios is defined by a ratio between any two of the five attenuation changes. A third calculator calculates the oxygen saturation based on the attenuation change ratios, while taking an oxygen saturation of arterial blood, an oxygen saturation of venous blood, a ratio between changes in arterial blood and venous blood, and a tissue term as four unknown values.