A forming device includes a gas supply part supplying a gas into a metal pipe material held and heated between a first die and a second die paired with each other. A driving mechanism moves at least one of the first die and the second die in a direction in which the dies are combined together. A first cavity part is formed between the first die and the second die to form the pipe part. A second cavity part communicates with the first cavity part to form the flange part. A flange forming member can be allowed to advance or retreat in the second cavity part, and forms the flange part. A controller controls the gas supply of the gas supply part, the driving of the driving mechanism, and the advance or retreat of the flange forming member.