PURPOSE: To easily calculate refractive index distribution within a short time, by using an optical system consisting of two convex lenses and arranging a filter having transmissivity distribution P2 in said system to measure the intensity distribution of light passing through said system.

CONSTITUTION: The light emitted from the end surface 3a of a fiber is allowed to be incident to a first convex lens 1 to be passed therethrough and succeedingly allowed to be incident to a second convex lens 2 to be passed therethrough to measure the luminous intensity distribution of emitted light at the post-focus 6 of the convex lens 2 while a filter having transmissivity distribution P2 is arranged to an X-X' surface and the light emitted from the end surface 3a of the fiber is passed through the convex lens 1 and the filter and subsequently passed through the convex lens 2 to similarly measure the luminous intensity distribution of the emitted light at the same position. In this case, when the filter is arranged, field distribution R(r) comes to P2H[R(r)] and is subsequently subjected to inverse Hankel transformation by the convex lens 2 to become H-1{P2H[R(r)]}. Two measured values thus obtained are substituted into a calculation formula and calculation is performed to make it possible to easily calculate refractive index distribution n(r) within short time.