To provide a heat treatment device having a gate oxide film of good dielectric breakdown strength.

A heat treatment device 30 is provided with switching valves 24A, 24B which are respectively arranged at an end of a gas inlet 20 and an end of a gas outlet 22 of a metal pipe 18, a branch pipe 32 branching from the metal pipe 18 between the switching valves 24A, 24B, and a vacuum exhaust mechanism 34 provided at the end of the branch pipe 32. The branch pipe 32 has a switching valve 36 in the vicinity of the metal pipe 18. During the transporting of a corrosive gas to a chamber of the heat treatment device 30, both switching valves 24A, 24B are opened. After the transporting is finished, the switching valves 24A, 24B are closed to operate the vacuum exhaust mechanism 34, and to open the switching valve 36 so as to subject the inside of the metal pipe 18 to vacuum suction. In this way, after the transportation of the corrosive gas, no corrosive gas will remain inside the metal pipe 18.