Workpieces (W) made up of vehicular body sides are transported along a production line (25) in an upright attitude, with the roof rail upward and the side sill downward. A plurality of hemming dies (27) for hemming the wheel arch part of the body sides (workpieces (W)) are disposed to the side of the production line (25), a hemming die (27) being held and caused to approach the workpiece (W) by a die transporting robot (29) so as to hem the workpiece, the die transporting robot (29) selecting a hemming die (27) of the plurality of hemming dies (27) that is appropriate for hemming a workpiece that arrives by transporting on the production line (25).