An intake valve assembly of an internal combustion engine. The intake valve assembly comprises a primary valve, a secondary valve mounted about the primary valve coaxially therewith, a primary valve spring for normally biasing the primary valve toward a closed position, a secondary valve spring for normally biasing the secondary valve toward a closed position, and a secondary valve lifter fixed to the primary valve and axially spaced from the secondary valve when both the primary and secondary valves are in closed positions. The secondary valve is operated mechanically by the secondary valve lifter and fluidly in response to pressure differential between the intake passage and the combustion chamber. The secondary valve is engageable with the primary valve through the secondary valve lifter after opening of the primary valve. The primary and secondary valve springs are normally contracted for continuously biasing the valves toward the closed position thereof.