PURPOSE:To detect the feed quantity, feed speed, or machining length, etc., of a belt conveyor, etc., on a conveyance path by detecting the displacement quantity of a speckle pattern by a speckle pattern diffracting circuit. CONSTITUTION:A controller part 12 has a speckle pattern analyzing circuit 15 to which the light receiving signal is inputted from a light receiving element 15 and the speckle pattern on the surface of a material 6 is detected from the light reception signal to operate the displacement quantity of the pattern, which is inputted to a comparison decision circuit 16 as the movement part of a material 6. The permissible range of the movement quantity of this material 6 can be inputted as a comparison difference setting (a) to the circuit 16, which compares the movement quantity of the material 6 inputted from the circuit 15 with the setting (a) and outputs the comparison result to an output means. A display part 17 fitted to the controller part 12 is usable as the output means.
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