To provide an oil injection device for an internal combustion engine capable of improving fuel economy by optimizing an injection amount of oil with a simple structure when the engine is in an operated state.

The oil injection device includes a cylinder block 5 having a cylinder 50, a cylinder bore 51 surrounding the cylinder 50 and having a cross hatch formed; and a piston 8 reciprocating in the cylinder 50. An oil hole 82a penetrating from an inner peripheral side surface 82b to an outer peripheral side surface 82c of the piston 8 and making oil flow therethrough is provided, and in a lower part separated from the piston 8, an injection nozzle 9 is provided for injecting the oil toward an opening of the oil hole 82a opened to the inner peripheral side surface 82b, and an injection amount of the oil is adjusted so that the injection amount of the oil injected from the injection nozzle 9 is the maximum just before the top dead center of the compression stroke of the piston 8, and the injection amount of the oil is the minimum in the stroke other than the compression stroke of the piston 8.