A plurality of gases are prevented from being mixed with each other in a gas supply path when forming a film on a substrate to be processed so as to prevent generation of particles to enable a stable and clean film formation. A film containing metal is formed on a substrate to be processed by supplying a first process gas containing the metal and a second process gas for reducing the first process gas to a process chamber. The first process gas is supplied from a first gas supply passage to the process chamber. The second process gas is supplied from a second gas supply passage to the process chamber and the second process gas is plasma-excited in the process chamber. A first reverse flow preventing gas consisting of H2 or He is supplied to the process chamber from the first gas supply passage when supplying the second process gas.