To provide a gas supply device for an internal combustion engine for efficiently accumulating gas pressure when gas at a pressure higher than that of an intake pressure is accumulated in a pressure accumulation part arranged for accumulating gas.

This gas supply device for accumulating compressed gas and supplying it to the internal combustion engine 1 is provided with the pressure accumulation part 10 for accumulating gas, an introduction passage 11 connecting the inside of a cylinder of the internal combustion engine and the pressure accumulation part mutually for introducing gas compressed inside the cylinder to the pressure accumulation part, and a passage opening/closing means 14 opening/closing the introduction passage. When gas compressed inside the cylinder is introduced to the pressure accumulation part, timing for opening the introduction passage opening/closing means is set to a time when the pressure inside the cylinder is substantially equalized to that inside the pressure accumulation part in a compression stoke of the internal combustion engine.