Provided is a manufacturing method of a sub-muffler outer cylinder (20, 220), the manufacturing method comprising pushing one push roll (10, 210) toward two receive rolls (11, 12, 211, 212) and then bending a plate-shape workpiece (W0, W20) to form the sub-muffler outer cylinder (20, 220) made of a cylindrical-shaped body. A push-in amount in a part (20f~20g, 220f~220g) of the workpiece corresponding to a corner part of the cylindrical-shaped body is made larger than a push-in amount in a part (20a~20e, 220a~220e) of the workpiece corresponding to a side part of the cylindrical-shaped body to form the cylinducal-shaped body (20, 220) having a polygonal cross-sectional shape.