The present invention relates to a CO_2 injection controlling method and apparatus comprising: a CO_2 supply unit for supplying CO_2, which is stored in a CO_2 tank, into an injection tube connected to an endoscope; an injection button provided at the injection tube to be opened and closed, and controlling the supply of CO_2 into the human body through the injection tube; a pressure sensor for sensing the injection pressure of CO_2 supplied through the injection tube; a storage unit for storing the injection pressure and the injection flow at an injection mode in which CO_2 is injected into the injection tube since the injection button is closed, the atmospheric pressure and the atmospheric flow at a atmospheric mode in which CO_2 is released into the atmosphere since the injection button is opened, the injection pressure at a first transition mode when the injection mode is converted into the atmospheric mode, and the injection pressure at a second transition mode when the atmospheric mode is converted into the injection mode, separately; and a controller for controlling the CO_2 supply unit to supply CO_2 at the atmospheric pressure and the atmospheric flow if the injection pressure at the first transition mode is sensed by the pressure sensor, and controlling the CO_2 supply unit to supply CO_2 at the injection pressure and the injection flow if the injection pressure at the second transition mode is sensed by the pressure sensor.