PURPOSE:To find a scanning position by deciding that the intensity of reflected light and the phase difference of the reflected light from a laser modulating signal are within specific ranges, and also measure the two-dimensional shape of a steel plate end part by using information, etc., obtained from a movement extent signal. CONSTITUTION:The output light of a laser 4 scans breadthwise on the surface of a steel plate 1 as small spot light 8. Part of irregularly reflected laser light is converged by a parabolic mirror 6 again and guided out as a photodetected light beam 10 by a mirror 9. The photodetected light beam 10 is incident on a photoelectric converting element 13 and amplified by a high frequency amplifier 14. The amplified output signal is led to a level decision signal 18 to obtain its phase difference from the output signal of a high frequency oscillator 2 and a decision device 19 generates the presence/absence signal of a steel plate eventually by the output of a phase decision device 16. A scanning position computing element 21 performs arithmetic on the basis of the steel plate presence/ absence signal and the angle detection signal 20 of a rotary mirror 5 and a shape arithmetic circuit 23 obtain the shape measured value 24 of the final head and tail of the steel plate 1 from the output of a detector 22 for the movement extent of a roll 25 for moving the steel plate.
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