A side gas injector for a plasma reaction chamber is provided. The side gas injector includes a circular distribution plate and a cover plate. The circular distribution plate includes an injection hole for injecting a reaction gas and a distribution channel part for distributing the reaction gas such that the reaction gas introduced from the injection hole can be radially simultaneously jetted in a plurality of positions along an inner circumference surface of the distribution plate. The cover plate is coupled to a top of the distribution plate and seals a top of the distribution channel part.