A brain function measurement device for performing measurement by functional near-infrared spectroscopy, comprising: a measurement unit configured to perform optical emission and detection in an alternative manner and in which a first probe, a second probe and a third probe each having an optical attenuator provided at a distal end thereof are supported by a holder and respectively disposed at the vertexes of a regular triangle; and a control unit configured to: initialize transmittances of all of the optical attenuators so that an amount of light emitted from the three probes becomes a desired value read from a storage unit of the brain function measurement device or supplied to the control unit from an input unit; identify, among three channels to be measured by the three probes, a first channel having a maximum light amount obtained through detection; perform detection in the second and the third channels (C31, C23) respectively positioned between the first probe and the second probe which are disposed at both ends of the first channel and the remaining third probe to detect which of the second and third channels (C31, C23) has a smaller light amount; adjust the transmittance of the optical attenuator provided for the first or the second probe (PR1, PR2) disposed at one end of one of the second and the third channels (C31, C21) in which the smaller light amount has been detected, so that light amounts detected in the second and the third channels (C31, C23) become equal; adjust the transmittance of the optical attenuator provided for the third probe so that a light amount detected in the second or the third channel (C31, C23) becomes equal to a light amount detected in the first channel; and perform measurement by functional near-infrared spectroscopy in a state in which all of the adjusted transmittances are maintained. a measurement unit configured to perform optical emission and detection in an alternative manner and in which a first probe, a second probe and a third probe each having an optical attenuator provided at a distal end thereof are supported by a holder and respectively disposed at the vertexes of a regular triangle; and a control unit configured to: initialize transmittances of all of the optical attenuators so that an amount of light emitted from the three probes becomes a desired value read from a storage unit of the brain function measurement device or supplied to the control unit from an input unit; identify, among three channels to be measured by the three probes, a first channel having a maximum light amount obtained through detection; perform detection in the second and the third channels (C31, C23) respectively positioned between the first probe and the second probe which are disposed at both ends of the first channel and the remaining third probe to detect which of the second and third channels (C31, C23) has a smaller light amount; adjust the transmittance of the optical attenuator provided for the first or the second probe (PR1, PR2) disposed at one end of one of the second and the third channels (C31, C21) in which the smaller light amount has been detected, so that light amounts detected in the second and the third channels (C31, C23) become equal; adjust the transmittance of the optical attenuator provided for the third probe so that a light amount detected in the second or the third channel (C31, C23) becomes equal to a light amount detected in the first channel; and perform measurement by functional near-infrared spectroscopy in a state in which all of the adjusted transmittances are maintained.