A liquid medicine injection apparatus of the present invention is directed to improvement of a precise and detailed technology that can implement two functions of a high liquid medicine injection pressure and easy filling of the apparatus with a liquid medicine in a single apparatus without a separate additional unit, i.e., a technology for effectively adjusting an internal pressure of the apparatus depending on a liquid medicine filling mode and a liquid medicine injection mode. In the liquid medicine injection apparatus of the present invention, when an internal pressure of the apparatus exceeds a preset pressure due to a gas generated in a gas-generating unit of the apparatus, a switch member in the liquid medicine injection apparatus can operate as a pressure regulating valve, thereby constantly keeping the internal pressure of the apparatus at the preset pressure.
