To provide a gas injection device that can discharge a reversal-flow leaking liquid.

This gas injection device enabling reversal-flow leaking liquid discharge, comprises a chamber, a gas inlet conduit, a gas outlet conduit, and a liquid discharging conduit, wherein the gas outlet conduit and the liquid discharging conduit are provided with a check valve, respectively. When a gas is passed to the chamber through the gas inlet conduit, the check valve of the gas outlet conduit is open for the gas to be passed into a liquid and the check valve of the liquid discharging conduit is closed. While injection of the gas into the liquid is stopped, the check valve of the gas outlet conduit is closed to prevent the liquid from the reversal flow. The small amount of reversal-flow leaking liquid accumulated in the chamber can open the check valve of the liquid discharging conduit to discharge the leaking liquid from the chamber.