There is provided a stapler which can perform a stapling operation with smaller force. A connected staple assembly cassette eliminates a metallic feed spring pressing the connected staple assembly, and instead, a reverse movement stop feeder is mounted. A mount sensor for sensing the mounting of the connected staple assembly cassette is provided for the stapler. In addition, a pusher piece is provided in order to follow up the reverse movement stop feeder in the connected staple assembly cassette with the sensing of the mount sensor and to push forward the reverse movement feeder. A magazine support spring for returning the staple mount magazine to the original position is eliminated, and the staple mount magazine can be returned to the original position by the released operation handle. During the advancing of the front staple in the staple lowering slit, the next staple can be blocked from advance movement.