An apparatus including a vacuum chamber having a substrate holding unit that holds a substrate and a plasma electrode facing the substrate, a first gas supply unit that supplies a H2 gas to the vacuum chamber at a constant flow rate, a second gas supply unit that opens or closes a valve to turn on or off the supply of a SiH4 gas, a high-frequency power source that applies a high frequency voltage to the plasma electrode, a shield box that is connected to a ground so as to surround the plasma electrode outside the vacuum chamber, and a control unit that controls the valve such that the SiH4 gas is periodically supplied to the vacuum chamber and modulates the amplitude of high frequency power in synchronization with the opening or closing of the valve, and the valve is provided in the shield box.