A stapler comprising: a clincher unit having a clincher part configured to bend leg portions of a staple and provided on one end portion-side; a magazine unit having a magazine in which a staple bundle consisting of staples connected to each other is loaded, and supported on the other end portion-side of the clincher unit to be rotatable about a fulcrum shaft portion as a fulcrum with respect to the clincher unit; a driver unit having a driver configured to strike out a staple from the magazine and provided on one end portion-side, the other end portion-side of the driver unit being supported to be rotatable about the fulcrum shaft portion as a fulcrum with respect to the clincher unit; a handle unit connected to the driver unit by a boosting pin provided between the fulcrum shaft portion and the driver, and configured to move the driver unit toward the clincher unit for opening and closing; a camshaft portion provided to any one of the clincher unit and the handle unit; a cam groove portion provided to the other of the clincher unit and the handle unit, configured to permit the camshaft portion to enter, and having one open end portion in a direction, in which the clincher unit and the handle unit separate from each other, to enable the camshaft portion to be inserted/pulled out; and a guide portion configured to restrict a movement path of the handle unit with respect to the clincher unit in operations of opening and closing the handle unit and the driver unit with respect to the clincher unit and the magazine unit, wherein a fulcrum axis of a rotation operation of the handle unit is moved as the cam groove portion is guided by the camshaft portion in accordance with the rotation operation of the handle unit, wherein the fulcrum axis is a virtual fulcrum axis when displacement of the handle unit is regarded as the rotation operation, characterized in that the cam groove portion has a first section (50d) and a second section (50d), wherein, in an operation for holding a sheet between the clincher unit and the magazine unit, the first section (50d) is in contact with the camshaft portion and the virtual fulcrum axis is at a given position near the fulcrum shaft portion, wherein, in an operation for causing the staple to penetrate through and to bind a sheet bundle, the second section (50d) is in contact with the camshaft portion and the virtual fulcrum axis moves from the given position toward the boosting pin which is a force point to which force is applied by the handle unit. a clincher unit having a clincher part configured to bend leg portions of a staple and provided on one end portion-side; a magazine unit having a magazine in which a staple bundle consisting of staples connected to each other is loaded, and supported on the other end portion-side of the clincher unit to be rotatable about a fulcrum shaft portion as a fulcrum with respect to the clincher unit; a driver unit having a driver configured to strike out a staple from the magazine and provided on one end portion-side, the other end portion-side of the driver unit being supported to be rotatable about the fulcrum shaft portion as a fulcrum with respect to the clincher unit; a handle unit connected to the driver unit by a boosting pin provided between the fulcrum shaft portion and the driver, and configured to move the driver unit toward the clincher unit for opening and closing; a camshaft portion provided to any one of the clincher unit and the handle unit; a cam groove portion provided to the other of the clincher unit and the handle unit, configured to permit the camshaft portion to enter, and having one open end portion in a direction, in which the clincher unit and the handle unit separate from each other, to enable the camshaft portion to be inserted/pulled out; and a guide portion configured to restrict a movement path of the handle unit with respect to the clincher unit in operations of opening and closing the handle unit and the driver unit with respect to the clincher unit and the magazine unit, wherein a fulcrum axis of a rotation operation of the handle unit is moved as the cam groove portion is guided by the camshaft portion in accordance with the rotation operation of the handle unit, wherein the fulcrum axis is a virtual fulcrum axis when displacement of the handle unit is regarded as the rotation operation, characterized in that the cam groove portion has a first section (50d) and a second section (50d), wherein, in an operation for holding a sheet between the clincher unit and the magazine unit, the first section (50d) is in contact with the camshaft portion and the virtual fulcrum axis is at a given position near the fulcrum shaft portion, wherein, in an operation for causing the staple to penetrate through and to bind a sheet bundle, the second section (50d) is in contact with the camshaft portion and the virtual fulcrum axis moves from the given position toward the boosting pin which is a force point to which force is applied by the handle unit.