An apparatus for manufacturing semiconductor devices having a gas mixer includes a gas supply and a reaction chamber, and the gas supply includes an upper gas mixer, an intermediate gas mixer disposed under the upper gas mixer, a lower gas mixer disposed under the intermediate gas mixer, a first gas supply pipe which is disposed on an upper portion of the upper gas mixer and supplies a first gas to the upper gas mixer, a second gas supply pipe which is disposed on an upper end portion of a side surface of the upper gas mixer and supplies a second gas to the upper gas mixer, and a third gas supply pipe which is disposed on a side surface of the intermediate gas mixer and supplies a third gas to the intermediate gas mixer.