A blood oxygen saturation measuring apparatus includes: a light emitter adapted to emit, to living tissue of a subject, at least two light beams which have different wavelengths from each other； a light receiver adapted to receive the light beams which are transmitted through or reflected from the living tissue, the light receiver which converts the received light beams to electric signals corresponding to receiving intensities of the light beams, respectively； an extractor which extracts frequency components corresponding to changes of the receiving intensities according to a variation of a blood volume of a vein in the living tissue due to respiration of the subject, from the electric signals, respectively； an attenuation ratio calculator which calculates a first attenuation ratio based on amplitudes of the extracted frequency components； and an oxygen saturation calculator which calculates a first blood oxygen saturation in the vein based on the first attenuation ratio.
