Abstract: A plurality of radiation receiving elements disposed outside a pipe at equal distances from the axis thereof so that radiation from a radiation source is received through the pipe wall. As the thickness of the pipe wall varies, the received radiation varies accordingly. If the received radiation fluctuates in excess of the allowable range, the thickness of the pipe wall is judged to be outside the allowable range. When the fluctuation of the received radiation is within the allowable range, the thickness of the pipe wall is within the preset tolerances.