A first mass flow controller is provided in an inert gas pipe for feeding nitrogen gas. A second mass flow controller is provided in a reactive gas pipe for feeding ammonia. A joint pipe communicatively connects a joint portion of the inert gas pipe and the reactive gas pipe to a chamber for treating a semiconductor wafer. The joint pipe is provided with a mass flowmeter. A detector detects gas leakage by comparing a total value of flow rates of nitrogen controlled by the first mass flow controller and of ammonia controlled by the second mass flow controller with a measurement value of a flow rate of a treatment gas, obtained by the mass flowmeter.