Disclosed is a probe for biomeasurement by use of light capable of adjusting positions of incident points and detection points without changing the distance between the incident point and the detection point in accordance with the size of a subject's head, and an optical bioinstrumentation for living body using the probe. The distances between the incident points and the detection points are approximately the same. A gap is formed by removing part of connecting members, rotatable about the incident point and the detection point, around the subject head top portion. The size of the probe is changed by changing the distance between the incident points or the detection points around the gap.