PURPOSE: To measure the intensity distribution of light of a single-mode optical fiber, by moving an optical fiber probe expanding the end face of the optical fiber with a lens to measure the light intensity distribution at positions.

CONSTITUTION: Output of a light source 1 is introduced with a pig-tail fiber 2 with a one end connector, which connected to a fiber 4 to be measured with a V-groove connector 3. The other end of the fiber 4 is set on a stage of an XYZ fine adjusting base 5 and monitored with a eye-piece lens 13 to focus. An optical fiber probe 9 is set at the position where an image is formed with lenses 7 and 8 and the light receiving end side thereof is moved in X and Y axes with a fine adjusting base 6 controlled with a CPU12 to pick up light at positions of an expanded image in a spot manner sequentially to detect. The other end of the probe 9 is connected to a detector 10 and light detected with this device is inputted into the CPU12 through an amplifier 11 to determine the spot size by a computation processing.