In a plate thickness detector for a bending machine causing a punch to make a relative stroke and bending a workpiece mounted on an upper surface of a die cooperatively by the punch and the die, a displacement gauge (17) is provided in the die, is always urged upward from a V-groove of the die, and measures a distance from the upper surface of the die to a lower surface of the workpiece, and ram position detection means (11) detects a relative stroke quantity of the punch to the die. The punch is caused to bend the workpiece from a position away from the die by a reference inter-blade distance. A plate thickness arithmetic operation section (35) inputs the relative stroke quantity of the punch at a point at which descent of the workpiece is detected by the displacement gauge or a predetermined point after the point, inputs the displacement quantity of the displacement gauge at this time using ram position detection means (11), and detects the plate thickness of the workpiece by subtracting the detected relative stroke quantity from the reference inter-blade distance and adding the displacement quantity detected by the displacement gauge to the subtraction result. IMAGE
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