A processing method includes a gas having a Si—CH3 bond supplied into a processing chamber after a target substrate to be processed is loaded into the processing chamber; and a silylation process performed on the target substrate. The internal pressure of the chamber by the supply of the gas having the Si—CH3 bond and the gas supply time are set to be within ranges where the silylation process can be performed while the internal pressure of the chamber is decreased to reach an eligible pressure level where the wafer can be unloaded after the internal pressure of the chamber is increased up to a preset pressure by the supply of the gas.