A gas injector includes first and second gas introduction passages extending in a first direction toward a central axis of a process chamber respectively, a first bypass passage extending from the first gas introduction passage in a second direction that is substantially perpendicular to the first direction, a second bypass passage extending from the second gas introduction passage in a reverse direction to the second direction, a first distribution passage isolated from the first bypass passage in the first direction and extending from an outlet of the first bypass passage in the reverse direction to the second direction, a second distribution passage isolated from the second bypass passage in the first direction and extending from an outlet of the second bypass passage in the second direction, and a plurality of spray holes in an outer surface of the first and second distribution passages and configured to spray the process gas.