PURPOSE: To minimize the heating of an optical fiber by sampling the peripheral part of a luminous flux along a circle which is a core axis for conforming the luminous flux axis to the core axis of the optical fiber, and analyzing the angle distribution of intensity of the corresponding luminous flux by sample collecting and detecting means for detecting the luminous flux.

CONSTITUTION: The beam from a laser 6 is focused to the incident surface of an optical fiber 4 by a lens 8. The peripheral part of the focused beam is never advanced into the fiber 4. The peripheral part is shielded by the conical surface of a mirror 24. The point where the light is shielded is produced by a truncated conical hole 28. This sample part is reflected by the mirror 24 and made into a horizontal optical beam 50, which is then converted into a tubular beam 52 by the reflection by a mirror 26. The beam 52 is decomposed into an optical sample by a thin plate mirror 34, and detected by a photo detector 44. This signal and the signal of a synchronous assembly 48 are received by a proper means 54, whereby the angle distribution of light intensity not incident on the fiber 4 can be analyzed, and the centering of focusing of the optical beam can be checked.