To provide a heat treatment apparatus that can improve the flow velocity distribution of a gas supplied into a chamber and can control the disturbed temperature distribution of a substrate.

The gas piping 85 is connected to a first buffer space 81, at a side opposite to the side where a gas pipe 83 is connected. The base end section of the gas piping 85 is connected to a nitrogen gas supply source 87. A valve 86 is inserted at an intermediate part of the gas piping 85. If the valve 86 is opened, nitrogen gas is supplied to the first buffer space 81 from the nitrogen gas supply source 85 via the gas piping 85. The nitrogen gas flowing into the first buffer space 81 is supplied to the second buffer space 82 via the gas pipe 83. The nitrogen gas flowing into the second buffer space 82 passes through a slit-shaped exhaust nozzle 84 and is supplied to a heat treatment space 6 inside the chamber 6.