The controller controls the gas supply of the gas supply unit so as to maintain a pressure in a metal pipe material at a first pressure when a gas is supplied into the metal pipe material which is formed into a pipe portion in a main cavity portion in a state where an upper die and a lower die are joined to each other. It is possible to prevent pressure drop in the pipe portion caused by cooling of the pipe portion due to a contact between the dies, and the pipe portion. Thus, it is possible to suppress a decrease in a force for pressing the pipe portion against the dies. It is possible to suppress a decrease in adhesion between the pipe portion and the dies when the metal pipe is formed, and it is possible to suppress occurrence of variations in hardenability in the pipe portion.