Blow-by gas oil separator for an internal combustion engine

Internal combustion engine having a blow-by gas passage allowing communication between a crank chamber and an intake passage comprising a separator for separating oil from the blow-by gas and returning the oil to the crank chamber through an oil drain passage, wherein the separator comprises a separation chamber and a valve mechanism provided in the lower part of the separation chamber and connected to the oil-drain passage, wherein said valve mechanism comprises first and second valve elements coupled to each other and a valve seat member defining valve seats adapted to cooperate with the respective valve elements, wherein at least either of the first and second valve elements being separate from the associated valve seat in response to a position of the coupled valve elements.