An oxygen saturation-measuring device (1A) for measuring oxygen saturation in a living body is provided with: a light-injecting unit (21) for injecting a first light and a second light of mutually differing wavelengths into a living body from a single light-injecting position； a light detection unit (22) for detecting the light intensities of the first light and the second light that have been propagated inside the living body at a single light-detecting position； and a CPU (14) for using the degrees of reduction in the first light and the second light in the living body that are obtained from the detection results of the light detection unit (22) in a modified Beer-Lambert law to calculate the oxygen saturation in the living body. As a result, an oxygen saturation-measuring device and an oxygen saturation-calculating method having only one each of the light-injecting position and the light-detecting position and capable of calculating oxygen saturation with good precision are achieved.
