An external auditory canal arm having a photo sensing element at the distal end thereof is inserted into an external auditory meatus. A contact piece 1b having a light emitting element is set to a location near an ear lobe on the surface of a skin of the temporal region of a subject. Light that is emitted from the light emitting element and transmitted through the superficial temporal artery region, is received by the photo sensing element. A variation of the quantity of light when passing through blood flowing through the superficial temporal artery, which is caused by light absorptive materials in the blood, is detected, to thereby obtain oxygen saturation, cardiac output and blood volumes, and the like in the blood.