PURPOSE: To detect the eccentricity and wall thickness distribution of a tubular material in excellent accuracy by emitting radioactive rays from a plurality of radiation sources which are arranged approximately orthogonally in the peripheral direction at different positions in the tubular material.

CONSTITUTION: Radiation sources 1c and 1d are placed on both sides of a radiation source 1a. In addition, a radiation source 1b is arranged in a horizontal direction crossing the center O of a tubular material P. Three detectors 2, 2 and 2 are respectively placed opposite to the radiation sources 1a and 1b around the material P, and one detector 2 is repectively placed opposite to the radiation sources 1c and 1d. The detectors 2 to 2 receiving the radioactive rays 31 to 32 output signals depending on the amount of transmitting ray, and the signals are given to an amplifier 5 for amplification. Then they are converted into digital signals by an A/D convertor 6 and are given to a computer 7, and as a result the computer 7 computes the wall thickness distribution of the material P, then it outputs the result to a host computer.