A substrate processing method includes a first expanding step, a first gas supplying step, a first plasma processing step, and a first power stopping step. The first expanding step increases the volume of a gas diffusion chamber. The first gas supplying step supplies a first gas into the gas diffusion chamber. The first plasma processing step supplies radio-frequency power from a radio-frequency power supply to generate plasma in a processing chamber accommodating a substrate and reduces the volume of the gas diffusion chamber. The first power stopping step stops the supply of the radio-frequency power after the first plasma processing step.