To certainly detect the clogging of a staple.

In a stapler in which a U-shaped staple S supplied to a driving part at the front end of a magazine 1, is driven by a driver plate 8 so as to bind members 10 to be bound, each sensor 12 is arranged in a position where is at an end on the staple S driving side of the driving part 7 and each leg Sa of the staple S is passed through when driven, and whether or not each leg Sa of the staple S remains in the driving part 7 when the driver plate 8 is returned, is detected by each sensor 12.