To easily and speedily attach and detach a rotary cutter.

A plurality of keyholes 5 are formed in a rotary cutter 1 in the same direction on the same circumference along a peripheral direction of the rotary cutter 1. Guide pins 8 to engage the keyholes 5 are formed on a lower surface of a rotor plate 4 with head parts 8a down as many as the keyholes 5 to correspond to the keyholes 5. The head parts 8a are formed a little smaller than a large-diameter hole 5a of the keyhole 5 and also larger than a groove width of a cut 5b. A shaft part 8b is formed having its diameter a little smaller than the groove width of the cut 5b and also a little longer than a thickness of the rotary cutter 1 so as to hold the rotary cutter 1 with the head 8a and the rotor plate 4. A stopper pin 9 which enters the large-diameter hole 5a when the shaft part 8b is engaged with the cut 5b to prevent the rotary cutter 1 from moving in the peripheral direction is provided to the rotor plate 4 to be always elastic in a downward direction.