Film forming apparatus and film forming method

A film forming method comprising:

• placing a substrate in a reaction chamber;
• introducing a first source gas containing a component of a first film into a gas supplier facing the substrate and having gas supply holes provided thereon through a source-gas introducing line communicated with the gas supplier, and supplying the first source gas into the reaction chamber from the gas supplier through the gas supply holes, to form the first film on the substrate;
• stopping introduction of the first source gas and discharging the first source gas from the source-gas introducing line along with a vent gas through a vent line connected to the source-gas introducing line via a valve provided on the source-gas introducing line;
• and
• introducing a replacement gas into the gas supplier, discharging the replacement gas along with a first remaining source gas which is the first source gas remaining in the gas supplier from the gas supplier through a discharge line communicated with the gas supplier, and controlling one of an introduction flow rate of the replacement gas and a discharge flow rate of the first remaining source gas and the replacement gas to be a flow rate corresponding to the other flow rate by a mass flow controller, wherein
• the introducing the first source gas comprises causing the valve to be in a first state in which the valve opens a connection between the source-gas introducing line and the gas supplier and closes a connection between the source-gas introducing line and the vent line, and
• the stopping introduction of the first source gas comprises causing the valve to be in a second state in which the valve closes the connection between the source-gas introducing line and the gas supplier and opens the connection between the source-gas introducing line and the vent line to discharge the source gas from the source-gas introducing line along with the vent gas through the vent line.