In one embodiment, a clean gas supply mechanism includes a supply path having an upstream end provided with a gas intake port, and a downstream end connected to a treatment chamber. The supply path is provided therein with a fan, and a filter disposed upstream of the fan. A return path branches from the supply path at a first position downstream of the fan and upstream of the treatment chamber and is connected to the supply path at a second position downstream of the filter and upstream of the fan. A switching mechanism selectively switches between a first state where the clean gas flows into the treatment chamber, and a second state where the clean gas returns to the supply path through the return path.