In one embodiment, a gas supply line is connected to a chamber of a substrate processing apparatus. A vaporizer is connected to the gas supply line. A flow rate controller is connected to the gas supply line in parallel with the vaporizer through a secondary valve. A primary valve is provided on a primary side of the flow rate controller. A method of the embodiment includes supplying a processing gas to the chamber from the vaporizer through the gas supply line in a state in which the primary valve is closed, the secondary valve is opened, and an exhaust device is operated to set a pressure of the chamber to a predetermined pressure and determining a time-average value of a measurement value obtained by a pressure sensor of the flow rate controller while the supplying the processing gas is performed.