PURPOSE: To enhance measuring accuracy of displacement so as to improve accuracy of detecting a two-sheet doubled condition by providing a detecting piece pressing device for pressing a detecting piece to a reference contact piece and a measuring device for directly measuring displacement of the detecting piece.

CONSTITUTION: A turning frame 13 is left as retracted at an angle of not interfering with lifting a plate material W, and when it is lifted to predetermined height, the turning frame 13 is turned to a side of the plate material W to hold it by a reference contact piece 18 and a detecting piece 19. Under this condition, displacement of the detecting piece 19 is measured by a measuring device 24. A holding thickness is detected by this displacement detection value to discriminate whether the plate material W is placed in a two-sheet doubled condition or not.