A light emitting unit emits first light and second light having different wavelengths. A photoreception unit outputs first and second signals depending on photoreception intensities of the first and second light transmitted through or reflected by biological tissue. A processing period setting unit extracts a signal cycle corresponding to the cardiac cycle for the first or second signal and sets a processing period in a first part dominantly affected by arterial blood flowing to the tissue or a second part dominantly affected by venous blood flowing from the tissue in the signal cycle. First and second change amount acquisition units obtain first and second change amounts corresponding to the attenuation change amounts of the first and second light from the first and second signals in the processing period. A concentration calculation unit calculates the concentration of light absorbing substance in blood from the first and second change amounts.
