Method for manufacturing vehicle structural member

A method for manufacturing a vehicle structure member by using a hollow extruded material that has a pair of side plates and a plurality of connecting plates connecting the side plates together and that has a quadrilateral closed section and a longitudinal shape, such that a width dimension of the vehicle structure member, which is a distance between the side plates, varies in a longitudinal direction of the vehicle structure member, the method comprising:

• an extrusion molding step of producing the hollow extruded material with the connecting plates each having a bent shape by extrusion molding;
• and
• a widening step of increasing the width dimension by increasing the distance between the side plates partially in the longitudinal direction so as to flatten the connecting plates having the bent shape wherein
• in the widening step, a through hole is formed in one of the side plates, a punch is inserted through the through hole, and with the one side plate being held, the punch is moved to contact the other side plate to move the side plates away from each other, so that the width dimension is increased.