A blood oxygen saturation measuring apparatus comprising: 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 (30, 230) adapted to extract 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 adapted to calculate a first attenuation ratio based on amplitudes of the extracted frequency components; an oxygen saturation calculator adapted to calculate a first blood oxygen saturation in the vein based on the first attenuation ratio; a determiner adapted to determine that the subject is in an obstructive apnea condition, when the first blood oxygen saturation in the vein shows a decreasing trend for a predetermined time period or longer, wherein the extractor (30, 230) is adapted to extract pulsation components corresponding to changes of the receiving intensities according to pulsation of an artery in the living tissue, from the electric signals, respectively, the attenuation ratio calculator is adapted to calculate a second attenuation ratio based on amplitudes of the pulsation components, the oxygen saturation calculator is adapted to calculate a second blood oxygen saturation in the artery based on the second attenuation ratio, and the determiner is adapted to determine that the subject is in a central apnea condition, when the amplitudes of the frequency components are smaller than a predetermined value for a predetermined time period or longer and the second blood oxygen saturation in the artery does not show a decreasing trend. 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 (30, 230) adapted to extract 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 adapted to calculate a first attenuation ratio based on amplitudes of the extracted frequency components; an oxygen saturation calculator adapted to calculate a first blood oxygen saturation in the vein based on the first attenuation ratio; a determiner adapted to determine that the subject is in an obstructive apnea condition, when the first blood oxygen saturation in the vein shows a decreasing trend for a predetermined time period or longer, wherein the extractor (30, 230) is adapted to extract pulsation components corresponding to changes of the receiving intensities according to pulsation of an artery in the living tissue, from the electric signals, respectively, the attenuation ratio calculator is adapted to calculate a second attenuation ratio based on amplitudes of the pulsation components, the oxygen saturation calculator is adapted to calculate a second blood oxygen saturation in the artery based on the second attenuation ratio, and the determiner is adapted to determine that the subject is in a central apnea condition, when the amplitudes of the frequency components are smaller than a predetermined value for a predetermined time period or longer and the second blood oxygen saturation in the artery does not show a decreasing trend.