PROBLEM TO BE SOLVED: To accurately detect a motion of a living body, which is a cause of hampering photometry of which principle is used for medical measurement.SOLUTION: Acquired is the intensity of a first intensity signal corresponding to the intensity of a first light beam containing a first wavelength λtransmitted through or reflected from a tissue of a living body and/or the intensity of a second intensity signal corresponding to the intensity of a second light beam containing a second wavelength λtransmitted through or reflected from the tissue. The intensity ratio between the first intensity signal and the second intensity signal is acquired. A motion of the living body is detected based on the intensities and the intensity ratio. The ratio between the absorbance of water in the first wavelength λand the absorbance of water in the second wavelength λis deviated from 1. The absorbance of oxygenated hemoglobin in the first wavelength λis higher than the absorbance of water in the first wavelength λ. The absorbance of oxygenated hemoglobin in the second wavelength λis higher than the absorbance of water in the second wavelength λ. The ratio between the absorbance of oxygenated hemoglobin in the first wavelength λand the absorbance of oxygenated hemoglobin in the second wavelength λcan be approximated by 1.SELECTED DRAWING: Figure 3【課題】フォトメトリの原理を用いつつも、当該フォトメトリを阻害する一因である生体の体動を正確に検出する。【解決手段】生体の組織を透過あるいは反射した第一波長λ１を含む第一の光の強度に対応する第一強度信号の強度と、当該組織を透過あるいは反射した第二波長λ２を含む第二の光の強度に対応する第二強度信号の強度の一方が取得される。他方、第一強度信号と第二強度信号の強度比が取得される。当該強度と当該強度比に基づいて、当該生体の体動が検出される。第一波長λ１における水の吸光度と第二波長λ２における水の吸光度の比率は１から乖離している。第一波長λ１における酸化ヘモグロビンの吸光度は、第一波長λ１における水の吸光度よりも高い。第二波長λ２における酸化ヘモグロビンの吸光度は、第二波長λ２における水の吸光度よりも高い。第一波長λ１における酸化ヘモグロビンの吸光度と第二波長λ２における酸化ヘモグロビンの吸光度の比率は１で近似できる。【選択図】図３
