Gas supply method and film forming method

A film forming method for forming a metal film by repeating a raw material gas supply step of temporarily storing a raw material gas generated by vaporizing a raw material accommodated in a raw material container inside a buffer tank together with a carrier gas and subsequently supplying the raw material gas into a processing container, and a reducing gas supply step of supplying a reducing gas for reducing the raw material gas into the processing container, comprising:

• a first film forming process of forming the metal film on a first substrate by repeating the raw material gas supply step and the reducing gas supply step;
• a storing process of storing a first internal pressure of the buffer tank in the raw material gas supply step of the first film forming process;
• a depressurizing process of depressurizing an interior of the buffer tank;
• an adjusting process of adjusting a second internal pressure of the buffer tank to the first internal pressure stored in the storing process by filling the buffer tank with the raw material gas and the carrier gas and exhausting a gas from the buffer tank after the depressurizing process;
• and
• a second film forming process of forming the metal film on a second substrate different from the first substrate by repeating the raw material gas supply step and the reducing gas supply step after the adjusting process.