A method for removing water molecules from a vacuum chamber for carrying out a process on a target object in vacuum includes the steps of introducing into the vacuum chamber a water molecule removal gas including at least a reduction gas which reduces the water molecules to produce hydrogen molecules and a halogen-based gas which reacts with the produced hydrogen molecules to produce acid, exhausting gases in the vacuum chamber measuring an amount of water molecules present inside the vacuum chamber, and determining whether or not the measured amount of water molecules is greater than or equal to a threshold value, wherein if the measured amount of water molecules is greater than or equal to the threshold value, the water molecule removal gas is introduced into the vacuum chamber in the introducing step.