In one embodiment, a vaporizer is connected to a chamber of a substrate processing apparatus through a gas supply line and a gas introduction port. An exhaust device is connected to the gas supply line. The substrate processing apparatus includes a pressure sensor that obtains a measurement value of a pressure of the gas supply line. A method according to the embodiment includes supplying a processing gas to the chamber from the vaporizer through the gas supply line, and monitoring a change of the measurement value obtained by the pressure sensor in a state in which supply of the processing gas to the gas supply line is stopped.