PURPOSE: To detect a sectional shape of a body to be measured without making any mechanical scan be detecting a radiation emitted in the breadthwise direction of the body to be measured by a multichannel radiation detector through the body to be measured and averaging detection signals from respective detection bodies.

CONSTITUTION: Multichannel X-ray detectors 25W27 are arranged in opposition to X-ray generators 22W24 arranged in the breadthwise direction of the body 21 to be measured and thickness detection signals of the respective detectors 25W27 are processed by a filter/plate thickness conversion part 41 to calculate the thickness of the objective body 21, which is displayed on a CRT display device 45 through a display processing part 44 under the control of a main control part 42. In this case, the conversion part 41 eliminates statistical variance among respective detection signals; each detection signal corresponding to the center part of the objective body 21 is shaped into a signal with the slow leading edge through a filter function with a large time constant and respective detection signals corresponding to both end parts are shaped into signals with steep leading edges.