Provided is a brain function measurement device capable of improving the reliability of brain function measurement using a simple configuration. The brain function measurement device includes light source probes LD2 and LD12 for irradiating the scalp of a test subject with light beams; linearly polarizing films P2 and P3 for polarizing the light beams emitted from the light source probes LD2 and LD12 in a first direction; a linearly polarizing film P1 for blocking components in the first direction of reflected light beams that are generated as the light beams emitted from the linearly polarizing film P2 and P3 are reflected by the hairs of the test subject; and a detection probe PD1 for detecting the intensity of a light beam that has passed through the linearly polarizing film P1.