PURPOSE: To enable a highly accurate measurement at a high speed by arranging an inspector so as to measure various structural parameters of an optical fiber in the same condition as when light is actually transmitted.

CONSTITUTION: A light emitting diode 5 projects one end face of an optical fiber 1' for inspection while a laser diode 6 is provided at the other end face thereof 1' to oscillate a laser light guided therethrough. A reflected image from the one end face is magnified by the optical system and taken by a photographing means 10. The video signal is converted into a digital signal with an A/D converter 14 and written into a frame memory FM with the writing operation controlled by a control circuit FMW. A microcomputer MCS uses the data written to determine a data on the brightness of the magnified image according to the specified signal processing sequence. The laser diode used for the light source enables the measurement of spot size and the refractive index distribution with the fiber set in the waveguide mode.