Provided is a multi-channel brain-functional near infrared spectroscopy device capable of easily levelling detector noise. The following control is performed: setting transmittances of all of optical attenuators to 1, setting intensities of all of light sources to a maximum light intensity within a safe irradiation range; determining the effective illumination intensity of each light source probe i as well as a minimum effective illumination intensity and a maximum effective illumination intensity thereof; changing the transmittance ai of each optical attenuator i on the light source probe side to [the minimum effective illumination intensity/effective illumination intensity of each light source probe i], thereby levelling the effective illumination intensity; and increasing the intensities of all of the light sources by W=[the maximum effective illumination intensity/the minimum effective incident efficiency] times, and determining the effective detection rate of each detector probe j as well as the minimum effective detection rate thereof; and performing control so as to level the effective detection rate by changing the transmittance aj of each optical attenuator j on the detector probe side to [the minimum effective detection rate/effective detection rate of each detector probe j], thereby equalizing detector noise among all the channels k.