While maintaining the detection sensitivity, it provides a measurement probe capable of performing diameter reduction. A measurement probe is detachably connected to the biological optical measurement device for performing optical measurement for a living body tissue, the illumination fiber for irradiating illumination light to body tissue, the reflected and / or scattered illumination light in the living body tissue comprising a plurality of receiving fibers for receiving the returning light, an illumination fiber and receiving fiber satisfies the following conditional expression. 0.10
