PURPOSE: To remove light entering a cladding part from a light source, and to measure the light loss of a body to be measured accurately by providing a fiber storage part, which is charged with a liquid or resin having a higher refractive index than the cladding of an optical fiber, and fixture parts in one body on a photodetector side.

CONSTITUTION: The coating of an optical fiber cable 14 as a body to be measured is removed nearly to the length of a cylinder 9 to expose an optical fiber 15, and this is run through holes formed in fixture parts 10 and 11. A liquid 16 (e.g. glycerol) or resin having a higher refractive index than the cladding of the optical fiber is injected through an injection opening to fill the cylinder 9. Then, a light source is connected to the other side of the body to be measured to measure its light loss. Cladding light entering the connection part for the messurement is transmitted through said liquid or resin in the cylinder 9 from the external circumference of the cladding to be removed. Thus, while the cladding light enters a photodetector 6, the light loss measurement is performed accurately.