An internal pressure-adjustable liquid medicine injection apparatus comprising: a cylinder having one end with a liquid medicine-flowing tube connected thereto and the other end with a gas-generating unit coupled thereto, wherein a liquid medicine flows into and out of the cylinder through the liquid medicine-flowing tube and the gas-generating unit has a gas-generating space in which a gas is generated; an injection piston hermetically movable in the cylinder and dividing an internal space of the cylinder into a liquid medicine storage space and a gas-supplied space, wherein the liquid medicine flows through the liquid medicine-flowing tube and fills the liquid medicine storage space and the gas generated in the gas-generating unit is supplied to the gas-supplied space; and characterized in having a switch member provided between the gas-supplied space and the gas-generating space to connect the gas-supplied space to an atmosphere upon filling of the liquid medicine storage space with the liquid medicine and to connect the gas-supplied space to the gas-generating space upon injection of the liquid medicine wherein the switch member is installed in the gas-generating unit to face toward the inside of the gas-supplied space; the switch member has an internal connection passage for connecting an external air entrance formed at one side of the gas-generating unit to the gas-supplied space, and a shut-off valve, the shut-off valve being hermetically moveable in the internal connection passage and dividing the internal connection passage into an external air inflow space into which external air can flow from the external air entrance, and a generated-gas inflow space into which the gas from the gas-generating unit flows; when the liquid medicine storage space is filled with the liquid medicine prior to the generation of the gas in the gas-generating unit, the shut-off valve is positioned at a location where external air from the external air entrance is caused to flow into the external air inflow space and the external air inflow space is in communication with the gas-supplied space, thereby equilibrating the internal pressure of the gas-supplied space to an atmospheric pressure; and when the liquid medicine is injected after the gas is generated in the gas-generating unit, the gas generated in the gas-generating unit flows into the generated-gas inflow space to move the shut-off valve toward the external air inflow space, thereby blocking inflow of the external air from the external air entrance into the external air inflow space so that a pressure of the gas generated by the gas-generating unit is exerted on the injection piston in the gas-supplied space. a cylinder having one end with a liquid medicine-flowing tube connected thereto and the other end with a gas-generating unit coupled thereto, wherein a liquid medicine flows into and out of the cylinder through the liquid medicine-flowing tube and the gas-generating unit has a gas-generating space in which a gas is generated; an injection piston hermetically movable in the cylinder and dividing an internal space of the cylinder into a liquid medicine storage space and a gas-supplied space, wherein the liquid medicine flows through the liquid medicine-flowing tube and fills the liquid medicine storage space and the gas generated in the gas-generating unit is supplied to the gas-supplied space; and characterized in having a switch member provided between the gas-supplied space and the gas-generating space to connect the gas-supplied space to an atmosphere upon filling of the liquid medicine storage space with the liquid medicine and to connect the gas-supplied space to the gas-generating space upon injection of the liquid medicine wherein the switch member is installed in the gas-generating unit to face toward the inside of the gas-supplied space; the switch member has an internal connection passage for connecting an external air entrance formed at one side of the gas-generating unit to the gas-supplied space, and a shut-off valve, the shut-off valve being hermetically moveable in the internal connection passage and dividing the internal connection passage into an external air inflow space into which external air can flow from the external air entrance, and a generated-gas inflow space into which the gas from the gas-generating unit flows; when the liquid medicine storage space is filled with the liquid medicine prior to the generation of the gas in the gas-generating unit, the shut-off valve is positioned at a location where external air from the external air entrance is caused to flow into the external air inflow space and the external air inflow space is in communication with the gas-supplied space, thereby equilibrating the internal pressure of the gas-supplied space to an atmospheric pressure; and when the liquid medicine is injected after the gas is generated in the gas-generating unit, the gas generated in the gas-generating unit flows into the generated-gas inflow space to move the shut-off valve toward the external air inflow space, thereby blocking inflow of the external air from the external air entrance into the external air inflow space so that a pressure of the gas generated by the gas-generating unit is exerted on the injection piston in the gas-supplied space.