PURPOSE: To form a calibrating reflector for verifying an optical measuring system having an optical fiber sonde in order to measure, e.g. the oxygen saturation raio of blood so as to hold relatively high accuracy, i.e., to expose only to the relatively low troubles.

CONSTITUTION: A calibrating reflector has a casing 12, which is opened to one side, a reflector having the form of an opaque layer 14, wherein reflecting particles are embedded, and positioner 11 having one central opening port. An optical fiber sonde 10 is contained in the central opening port under the state wherein the specified friction resistance is present. A transparent disk 13 is provided between the front end part of the optical fiber sonde 10 and the reflecting layer 14. The front end part of the optical fiber sonde 10 can be bonded an that surface in this constitution.