An apparatus and method for fabricating a semiconductor device using a 4-way valve with improved purge efficiency by improving a gas valve system by preventing dead volume from occurring are provided. The apparatus includes a reaction chamber in which a substrate is processed to fabricate a semiconductor device; a first processing gas supply pipe supplying a first processing gas into the reaction chamber; a 4-way valve having a first inlet, a second inlet, a first outlet, and a second outlet and installed at the first processing gas supply pipe such that the first inlet and the first outlet are connected to the first processing gas supply pipe; a second processing gas supply pipe connected to the second inlet of the 4-way valve to supply a second processing gas; a bypass connected to the second outlet of the 4-way valve; and a gate valve installed at the bypass.