A brain-functional near infrared spectroscopy device, comprising: n light source probes i (where 1≤i≤n) and m detector probes j (where n+1≤j≤n+m) each arranged on a surface of a head; an optical attenuator i configured to guide light with a wavelength λ from each light source to each light source probe i, at a transmittance a i; an optical attenuator j configured to transmit light with the wavelength λ detected with each detector probe j to a measurement data unit, at a transmittance a j; and control means for processing detected data received by the measurement data unit to detect a brain function activity from a change in absorbance of measured light on the basis of spectral absorption properties of oxyhemoglobin and deoxyhemoglobin in N channels k (where 1≤k≤N) each including one of the light source probes i and one of the detector probes j, wherein: the control means performs the following in advance: setting transmittances of all of the optical attenuators to 1, setting intensities of all of the light sources to a maximum light intensity within a safe irradiation range, and determining an effective illumination intensity of each light source probe i as well as a minimum effective illumination intensity and a maximum effective illumination intensity thereof, and changing the transmittance a i of each optical attenuator i on the light source probe side to a value obtained by dividing the minimum effective illumination intensity by the 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 times (where W=the maximum effective illumination intensity/the minimum effective incident efficiency), 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 a j of each optical attenuator j on the detector probe side to a value obtained by dividing the minimum effective detection rate by the effective detection rate of each detector probe j, thereby equalizing dispersion of detector noise of oxyhemoglobin and dispersion of detector noise of deoxyhemoglobin among all the channels k. n light source probes i (where 1≤i≤n) and m detector probes j (where n+1≤j≤n+m) each arranged on a surface of a head; an optical attenuator i configured to guide light with a wavelength λ from each light source to each light source probe i, at a transmittance a i; an optical attenuator j configured to transmit light with the wavelength λ detected with each detector probe j to a measurement data unit, at a transmittance a j; and control means for processing detected data received by the measurement data unit to detect a brain function activity from a change in absorbance of measured light on the basis of spectral absorption properties of oxyhemoglobin and deoxyhemoglobin in N channels k (where 1≤k≤N) each including one of the light source probes i and one of the detector probes j, wherein: the control means performs the following in advance: setting transmittances of all of the optical attenuators to 1, setting intensities of all of the light sources to a maximum light intensity within a safe irradiation range, and determining an effective illumination intensity of each light source probe i as well as a minimum effective illumination intensity and a maximum effective illumination intensity thereof, and changing the transmittance a i of each optical attenuator i on the light source probe side to a value obtained by dividing the minimum effective illumination intensity by the 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 times (where W=the maximum effective illumination intensity/the minimum effective incident efficiency), and determining the effective detection rate of each detector probe j as well as the minimum effective detection rate thereof, and setting transmittances of all of the optical attenuators to 1, setting intensities of all of the light sources to a maximum light intensity within a safe irradiation range, and determining an effective illumination intensity of each light source probe i as well as a minimum effective illumination intensity and a maximum effective illumination intensity thereof, and changing the transmittance a i of each optical attenuator i on the light source probe side to a value obtained by dividing the minimum effective illumination intensity by the 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 times (where W=the maximum effective illumination intensity/the minimum effective incident efficiency), 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 a j of each optical attenuator j on the detector probe side to a value obtained by dividing the minimum effective detection rate by the effective detection rate of each detector probe j, thereby equalizing dispersion of detector noise of oxyhemoglobin and dispersion of detector noise of deoxyhemoglobin among all the channels k.