To measure the maximum theoretical numerical aperture of an optical fiber easily and accurately.

A detector 6 detects light from a lens 5 and an optical fiber 2 through an aperture forming member 3 having an opening 4 disposed on the exit side of the optical fiber 2. The center of the opening 4 is aligned with the optical axis of the optical fiber 2 and the light receiving part 9 of the detector 6. While varying the angle θ, i.e., one halt of the spreading angle of light passing through the inner wall at the opening from the center at the exit end 8 of the optical fiber 8, by varying the distance between the aperture forming member 3 and the exit end 8 of the optical fiber 8, the quantity of light emitted from the light source 1, passed through the optical fiber 2 and the opening 4 of the aperture forming member 3, and received by the detector 6 is detected. A data representing the relationship between the quantity it received light and sinθ is then determined and the maximum theoretical numerical aperture of the optical fiber is determined from the value of sinθ at a point where the increasing quantity of light begins to saturate.