OIL DRAIN STRUCTURE FOR OIL MIST SEPARATOR

An oil drain structure for an oil mist separator is provided with a check valve (10) that opens and closes an oil discharge hole (8) formed in a lower end portion of a drain pipe (5) of the oil mist separator. The check valve (10) includes: a disc-shaped valve body (11) which opens and closes the oil discharge hole (8) from an outside; a spherical valve head portion (12) fitted in a tubular portion (6) of the drain pipe (5) with a fine clearance (”D) between the valve head portion (12) and the tubular portion (6), the fine clearance (”D) serving as an orifice; and a rod-shaped stem portion (13) that extends through the oil discharge hole (8) and connects the valve body (11) and the valve head portion (12). When blow-by gas is likely to flow back due to a pressure difference between an intake system and an interior of an engine, the check valve (10) undergoes an upward force due to a pressure difference caused when the gas flows through the fine clearance (”D), and the check valve (10) is surely brought into a closed state. When oil is accumulated to a certain height, the check valve (10) descends to discharge the oil.