In an oil separator provided in a cylinder head cover of an internal combustion engine and equipped with a partition wall disposed between a blow-by gas inlet and a blow-by gas outlet and having through holes each serving as an orifice for increasing a flow velocity of blow-by gas flow, a collision plate is arranged adjacent to the partition wall so as to be opposed to each of the through holes, for separating oil mist from blow-by gases. At least one upstanding wall is disposed downstream of the collision plate in a manner so as to be adjacent to an opening defined between a lower end of the collision plate and the bottom face of a separator chamber, for recapturing oil mist, once-separated from the blow-by gases but rescattering along with the blow-by gas flow passing through the opening of the collision plate.