A surface treatment apparatus 100 includes a diluent gas supplier 1, a fluorine gas supplier 2, a mixer 5 which mixes a diluent gas with a fluorine gas, and a reactor 6 which treats a treatment target by using a mixed gas generated by the mixer 5. A diluent gas supplied from a diluent gas supplier is heated by a heater 8, and the heated diluent gas is mixed with a fluorine gas supplied from a fluorine gas supplier, in the mixer 5. The mixed gas is supplied to the reactor 6. The gas in the reactor 6 is supplied from the reactor 6 to flow paths 219, 220, 221, and 222 by an exhauster 207. The valves 223, 224, 225, and 226 are serially opened, so that the gas in the reactor 6 is supplied to a harm removal apparatus 208 while the gas flow rate is being adjusted by the flow paths 219, 220, 221, and 222.