A manufacturing method of a sub-muffler outer cylinder is provided. The manufacturing method includes pushing one push roll toward two receive rolls, and then bending a plate-shape workpiece to form the sub-muffler outer cylinder made of a cylindrical-shaped body. A push-in amount in a part of the workpiece corresponding to a corner part of the cylindrical-shaped body is made larger than a push-in amount in a part of the workpiece corresponding to a side part of the cylindrical-shaped body to form the cylindrical-shaped body having a polygonal cross-sectional shape.