PURPOSE: To eliminate the need for a rotating mechanism and the mechanism part of an expanding and reducing machine and to increase the inverval to a body to be checked, by measuring the irregular part of an end part of the body to be checked optically.

CONSTITUTION: When a beam emitted from a transmission-side glass fiber 5a strikes the irregular terminal part 2, no reflected wave reaches a photodetection part, but when it passes through the irreqular terminal part 2, its reflected wave is detected by a reception-side fiber 5b at any position on the circumference. This phenomenon is utilized to measure the length of the irregular terminal part 2. When the body to be checked varies in external diameter, a couple of rings 5a and 5b are moved forth and back (as shown by an arrow) to the measurement and the need for the expanding and reducing mechanism is eliminated. Thus, the length of the irregular terminal part 2 of a rod, tube, etc., is measured optically, so the interval between the body 1 to be checked and a sensor is increased and the measurement precision is improved.