PURPOSE: To enable the configuration of an optical fiber base material to be accurately measured without being affected by a difference in a refractive index by arranging the optical fiber base material to be measured in a container filled with a highly accurately adjusted refractive index adjusting oil and scanning a laser beam on the central axis of the base material.

CONSTITUTION: An optical fiber base material 7 is fixed (8) and a glass container 9 is filled with a refractive index adjusting oil 14 with a refractive index (n) represented by the accompanying expression. (wherein n1 designates the refractive index of the clad of the base material 7.) Then, an optical system is operated to scan a parallel laser beam 11 in a direction perpendicular to the central axis of the base material 7 and signals indicative of a received light (6b) are fed to an electric signal processor 19. After subjected to an averaging processing (19) therein, the signals are stored in a mini-CPU 20. Pulse signals are fed from the mini-CPU 20 to a pulse motor 15 to rotate a rotary shaft 13 by every 5° and the same measurement is conducted for the whole rotary angle (360° direction). When the result is displayed (21), an outer diameter, a core diameter and the like in the axial direction of the base material 7 can be obtained with a high accuracy.