A method for using a film formation apparatus for a semiconductor process forms a first atmosphere inside an upstream gas passage between a gas supply source of a halogen acidic gas and a flow rate controller. The first atmosphere is set for the halogen acidic gas to have an average molecular weight of 20 or more and 23 or less. Further, the using method supplies the halogen acidic gas from the gas supply source through the upstream gas passage having the first atmosphere thus formed and the flow rate controller, thereby supplying a cleaning gas containing the halogen acidic gas into a reaction chamber of the film formation apparatus. A by-product film deposited on an inner surface of the reaction chamber is etched and removed by use of the cleaning gas thus supplied.