A gas injection module includes a showerhead having first injection holes on a first region of the showerhead and second injection holes on a second region of the showerhead, the second region being outside the first region, a first distribution plate on the showerhead and having first and second upper passages respectively connected to the first and second injection holes, and a flow rate controller on the first and second upper passages of the first distribution plate. The flow rate controller reduces a difference in pressure within the first and second upper passages so that the gas may have similar flow rates within the first and second injection holes.