A gas supply system may include a first gas supply line, a second gas supply line, a circulation gas pipe, a gas purification unit, a first valve, and a second valve. The first gas supply line may include a first branching point at which the first gas supply line branches into a first branch connected to a first chamber and a second branch connected to a second chamber and the second gas supply line may include a second branching point at which the second gas supply line branches into a third branch connected to the first chamber and a fourth branch connected to the second chamber. A first portion of the first gas supply line upstream from the first branching point and a second portion of the second gas supply line upstream from the second branching point may be constituted by separate pipes from each other.