The invention provides optical technique for measuring metabolism in a living
body that can further reduce an error in measurement even if distance between irradiation
and detection is fixed. The living body is classified into measurement regions
and different measuring wavelengths are set according to the classification. For
example, the head is classified into four regions of a parietal region, a frontal
region, a temporal region and an occipital region and a wavelength according to
each tissue is set in a wavelength selecting system. In measurement, wavelengths
according to each measurement region are selected by multiple wavelengths-light
radiating means. Besides, for a region having large personal difference, premeasurement
is made using plural combinations of wavelengths and wavelengths are selected using
a calculated error in measurement as a criterion.
