A forming device comprising: a fluid supply unit that is disposed at an end portion of a metal pipe material and supplies a first fluid to an inside of the metal pipe material via an opening of the end portion, wherein the fluid supply unit includes a surrounding portion that surrounds an outer peripheral surface of the end portion and at which an annular groove portion is formed on an inner peripheral surface facing the outer peripheral surface, an annular sealing member disposed in the groove portion, characterized in that the fluid supply unit further includes an operating portion at which a pressurizing force that pressurizes the sealing member toward the outer peripheral surface is generated, the groove portion includes a first side surface which is a side surface on one side in a direction along a central axis of the inner peripheral surface and a second side surface which is a side surface on the other side, the sealing member is disposed to come into contact with each of the first side surface and the second side surface and partitions a space surrounded by the surrounding portion into an outer peripheral side space which is closer to an outer peripheral side in a radial direction of the inner peripheral surface than the sealing member in the groove portion and an inner peripheral side space which is closer to an inner peripheral side in the radial direction of the inner peripheral surface than the sealing member in the groove portion, a second fluid is supplied to the outer peripheral side space through the operating portion, and the groove portion is formed such that an inner peripheral side exposure area, which is an area of exposure of the sealing member with respect to the inner peripheral side space as seen in the radial direction of the inner peripheral surface from the central axis, becomes smaller than an outer peripheral side exposure area, which is an area of exposure of the sealing member with respect to the outer peripheral side space as seen in the radial direction of the inner peripheral surface from the central axis. a fluid supply unit that is disposed at an end portion of a metal pipe material and supplies a first fluid to an inside of the metal pipe material via an opening of the end portion, wherein the fluid supply unit includes a surrounding portion that surrounds an outer peripheral surface of the end portion and at which an annular groove portion is formed on an inner peripheral surface facing the outer peripheral surface, an annular sealing member disposed in the groove portion, characterized in that the fluid supply unit further includes an operating portion at which a pressurizing force that pressurizes the sealing member toward the outer peripheral surface is generated, a surrounding portion that surrounds an outer peripheral surface of the end portion and at which an annular groove portion is formed on an inner peripheral surface facing the outer peripheral surface, an annular sealing member disposed in the groove portion, characterized in that the fluid supply unit further includes an operating portion at which a pressurizing force that pressurizes the sealing member toward the outer peripheral surface is generated, the groove portion includes a first side surface which is a side surface on one side in a direction along a central axis of the inner peripheral surface and a second side surface which is a side surface on the other side, the sealing member is disposed to come into contact with each of the first side surface and the second side surface and partitions a space surrounded by the surrounding portion into an outer peripheral side space which is closer to an outer peripheral side in a radial direction of the inner peripheral surface than the sealing member in the groove portion and an inner peripheral side space which is closer to an inner peripheral side in the radial direction of the inner peripheral surface than the sealing member in the groove portion, a second fluid is supplied to the outer peripheral side space through the operating portion, and the groove portion is formed such that an inner peripheral side exposure area, which is an area of exposure of the sealing member with respect to the inner peripheral side space as seen in the radial direction of the inner peripheral surface from the central axis, becomes smaller than an outer peripheral side exposure area, which is an area of exposure of the sealing member with respect to the outer peripheral side space as seen in the radial direction of the inner peripheral surface from the central axis.