PURPOSE:To improve response performance and stability performance by emitting a plurality of lights differed in wavelength from the surface of the skin, detecting the reflected light or transmitted light signals to arithmetically operate the arterial blood oxygen saturation, and selecting the central value from the past value and present value of the calculated value. CONSTITUTION:A probe 1 has an infrared light emitting element, a red light emitting element, an infrared light receiving element, and a red light receiving element, the reflected lights or transmitted light signals are converted into electric signals by a head amplifier 2 to extract infrared AC signal and red AC signal from infrared DC signal and red DC signal. In a CPU 4, the pulse wave is detected from the AC signal by a pulse wave detecting part 5, and the pulse waveform is displayed on a display part 10 by a display control part 9. On the other hand, the DC signal is inputted to a SpO2 (arterial blood oxygen saturation) arithmetic part 6 together with the pulse wave detection signal, the SpO2 value determined here is transmitted to a comparative processing part 7 to determine a central value sa, the movement of the central value is averaged by a movement averaging part 8, and the result is displayed on the display part 10.
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