PURPOSE: To make it possible to discriminate a leading end cut, a trailing end cut or two piece cuts, by rotating an iron plate blade wheel by a cut from a crop shear so that a crop is surely detected by means of only one detector.

CONSTITUTION: A leading/trailing end crop cut by a crop shear 2 which is a cutter for a steel to be rolled, rotates an iron plate blade wheel 6 clcokwise or counterclockwise when it drops. The direction of this rotation is detected by a reversible pulse transmitter (detector) 7 and is discriminated by a controller 5. That is, the clockwise direction of the rotation indicates the leading end cut, but the counterclockwise direction indicates the trailing end cut. When no rotational signal is issued from the detector 7 even though a cut signal is delivered from the crop shear, two piece cuts are discriminated. According to the above- mentioned detector device, only one signal cable is required for the controller, and an environmental resistibility in a place with bad surroundings for installation may be aimed at being enhanced since no light is used, thereby a crop may surely be detected.