As the measurement probe which measures optically by input/output doing light from the point it is provided in (3), the illumination fiber (311), and the fiber unit which possesses the plural light absorbent fibers (310) it is, the fiber for illumination (311) and as for the fiber for plural detection, the crookedness possible soft section (331) with, the soft section (331) the advanced hard section of higher hardness (332) with, the soft section (331) with the advanced hard section (332) with it connects, being crooked, between the fiber which it adjoins by the thing which extends, the soft section (331) inThe pitch converter which changes the distance with the other fiber which it adjoins vis-a-vis distance, the advanced hard section (332) in (333) with, it has, when the soft section (331) the end and the advanced hard section side (332) the line segment and the advanced hard section which tie the end side (332) among the angles which the medial axis forms, most designating the angle which inclines as Deg_max the pitch converter (333) in, it fills up 1.0
