An optical measurement system having a main and a portable optical measurement devices. The portable optical measurement device is provided with a second case portable by the subject, C second light sending means for illuminating the subject, D second light receiving means receiving light emitted from the subject, a second holder worn on the head of the subject and having at least (C+D) through holes therein, a control unit acquiring measurement data relating to a brain activity while controlling the second light sending and receiving means, and a communication unit communicating with the main optical measurement device, that (C+D)>(A+B) is satisfied, and that the communication unit of the portable optical measurement device transmits the measurement data acquired by the control unit of the portable optical measurement device to the main optical measurement device.
