A forming device of forming a metal pipe having a pipe portion, comprising: a first die and a second die which are paired with each other, and constitute a first cavity portion configured to form the pipe portion and a second cavity portion (SC1, SC2) which communicates with the first cavity portion and is configured to form a flange portion of the metal pipe when the first and second dies (11, 12) are closed; a heating unit configured to heat the metal pipe; a drive mechanism configured to move at least one of the first die and the second die in a direction in which the dies (11, 12) are joined to each other; and a gas supply unit configured to supply a gas into a heated metal pipe material which is held between the first die and the second die, wherein each of the first die and the second die has flange forming surfaces (F1, F2, F3, F4) which face each other and constitute the second cavity portion (SC1, SC2), a protrusion portion (110A, 110B, 111A, 111B), which protrudes from one flange forming surface to the other flange forming surface, is formed on at least one of the flange forming surfaces (F1, F2, F3, F4) of the first die and the second die, the protrusion portion (110A, 110B, 111A, 111B) is formed inside an outer end portion of the flange forming surface (F1, F2, F3, F4), characterized in that the gas supply unit is configured to supply the gas into the heated metal pipe material when the pipe portion and flange portion are formed, the protrusion portion (110A, 110B, 111A, 111B) is configured to form, inside an outer end portion of the flange portion, a gap portion where inner surfaces (151, 152) of the flange portion facing each other are separated from each other and a proximity portion where the inner surfaces (151, 152) are close to each other compared to the gap portion. a first die and a second die which are paired with each other, and constitute a first cavity portion configured to form the pipe portion and a second cavity portion (SC1, SC2) which communicates with the first cavity portion and is configured to form a flange portion of the metal pipe when the first and second dies (11, 12) are closed; a heating unit configured to heat the metal pipe; a drive mechanism configured to move at least one of the first die and the second die in a direction in which the dies (11, 12) are joined to each other; and a gas supply unit configured to supply a gas into a heated metal pipe material which is held between the first die and the second die, wherein each of the first die and the second die has flange forming surfaces (F1, F2, F3, F4) which face each other and constitute the second cavity portion (SC1, SC2), a protrusion portion (110A, 110B, 111A, 111B), which protrudes from one flange forming surface to the other flange forming surface, is formed on at least one of the flange forming surfaces (F1, F2, F3, F4) of the first die and the second die, the protrusion portion (110A, 110B, 111A, 111B) is formed inside an outer end portion of the flange forming surface (F1, F2, F3, F4), characterized in that the gas supply unit is configured to supply the gas into the heated metal pipe material when the pipe portion and flange portion are formed, the protrusion portion (110A, 110B, 111A, 111B) is configured to form, inside an outer end portion of the flange portion, a gap portion where inner surfaces (151, 152) of the flange portion facing each other are separated from each other and a proximity portion where the inner surfaces (151, 152) are close to each other compared to the gap portion.