PURPOSE: To measure the quantity of absorbed light accurately by conducting light through optical fibers, and installing a reflecting mirror of a spherical shape by leaving a spacing opposite to the exit and incident ends of the optical fibers.

CONSTITUTION: Two pieces of optical fibers 1, 2 are disposed in such a way that their opening ends 1a, 2a are put adjacent to each other by leaving a mutual spacing L1, and the ends 1a, 2a service respectively as an exit end and an inciden end. A holding metal fitting 3 and a reflecting mirror of a spherical shape are installed opposite to the ends 1a, 2a by leaving a suitable isolating spacing L2. According to the above-mentioned constitution, the most of the light emitted at a diffraction angle from the fiber 1 is reflected by the mirror 4 and is made incident to the converged end 2a of the fiber 2; hence, there are virtually no coupling losses between the ends 1a, 2a and therefore the quantity of absorbed light is measured accurately.