A forming device includes: a first cavity portion in which a pipe portion is formed and a second cavity portion in which a flange portion is formed, the first cavity portion and the second cavity portion being formed between a first die and a second die; a flange adjusting member which is capable of being advanced into the second cavity portion and is capable of being retreated from the second cavity portion, and which adjusts a length of the flange portion; and a control unit. The control unit sequentially performs a first control of allowing the flange adjusting member to be advanced into the second cavity portion, a second control of allowing the gas supply unit to supply a gas so as to temporarily form the flange portion, and a third control of allowing the flange adjusting member to be retreated from the second cavity portion.