A press forming method is capable of improving material yield while avoiding the risk of dimensional error increase at product forming sections in blank material when carrying out press forming, and a plate material expansion device is used in this method. An end side of plate material for press forming is restricted by blank holding rollers, and pressure is applied to a middle location in the part that is restricted by a pressure roller. In this state, the plate material is transported in a rotating direction of the rollers being driven to continuously bend and extend the section receiving pressure. This bent and extended section is flattened with a flattening roller to obtain blank material wherein an end part is expanded.