PURPOSE: To make it possible to simply and quantitatively inspect the inner surface of a small caliber tube, by inserting a probe equipped with a light emitting fiber and a light receiving fiber in a metal tube.

CONSTITUTION: A probe 1 is inserted in a tube T to be gradually moved to the axial direction. With the movement of the probe 1 or in such a state that the probe 1 is stopped at a predetermined position, the light from a light emitter 3 is projected to the inner surface of the tube T from the light emitting end 21 of an inspection part 11 through a light emitting fiber 2. Hereupon, the light receiving ends 41, 51 of two light receiving fibers 4, 5 respectively receive beams of light reflected from the inner surface of the tube T to send the same to photoelectric converters 6, 7. The converters 6, 7 convert the beams of light received by the fibers 4, 5 to electric signals to send the same to a comparator 8. The comparator 8 compares both outputs from the converters 6, 7 to measure the relation thereof and the distance L between the inspection part 11 and the inner surface of the tube. The measured result obtained by the comparator 8 is displayed on a display device 9 and the state such as the reduction of the wall thickness or damage in the tube is inspected.