An oil drain structure for an oil mist separator including a drain pipe extending from the oil mist separator into the internal combustion engine, the drain pipe having an oil discharge hole extending through a lower end portion of the drain pipe along an axial direction of the drain pipe, and a check valve including a valve body disposed below the lower end portion of the drain pipe, the valve body acting to open and close the oil discharge hole from a side of a lower surface of the lower end portion of the drain pipe, a valve head portion fitted into the drain pipe with a fine clearance as an orifice and being moveable in the axial direction of the drain pipe, and a stem portion extending through the oil discharge hole to connect the valve body and the valve head portion with each other.