PURPOSE: To enable determination of the size of ruggedness in a pipe quantitatively, by forming a ring-shaped spot light image on the surface of the pipe to determine the diameter of a ring from a coordinate position of the spot light image taken.

CONSTITUTION: Light generated from a light generation source 21 of a pipe body inspection pig 10 is branched off with an optical fiber 22 to be outputted aslant outward as light spot from a corresponding light outgoing circuit 23. A ring-shaped spot light image formed on the pipe surface is taken with a camera 13 and read out sequentially with a control section 24 having a camera control function to be recorded sequentially on a VTR 25. Then, the coordinate position of the spot light image stored in an image memory is determined by a circle ratio computing means 33 to obtain the diameter of a ring from this coordinate position relationship. Then, a circle diameter ratio is obtained from an inner diameter data of a pipe body previously stored into a memory 35, thereby determining the size of a recess, namely, the width and depth of a defect by a defect value computing means 36 from the ratio.