An oil separator provided in combination with a cylinder head cover of an internal combustion engine. The oil separator includes a separator cover fixed to an inner surface of the cylinder head cover defining a space extending in a first direction perpendicular to axis of a camshaft in plan, between the separator cover and the cylinder head cover. The separator cover has an opening through which the space is opened to a valve operating chamber. A partition wall is provided to define in the space an inlet-side separator chamber and an outlet-side separator. The partition wall extends in a second direction parallel with the axis of the camshaft and being formed with a plurality of fine passages. Additionally, a plurality of projection walls project from a part of the inner surface of the cylinder head cover which faces the valve operating chamber through the opening.