Abstract: A flow rate control system for controlling the idling rpm of an engine of a vehicle to a predetermined level in accordance with the temperature of cooling water or outdoor air, so as to eliminate hunting occurring in the flow rate of air supplied to the engine at engine startup the system includes a first valve seat, adapted to come into and out of engagement with a first valve body of a flow rate control valve assembly, and an annular protrusion located on a portion of an inner peripheral surface on an inlet side of the first valve seat. A conical surface portion is located downstream of the annular protrusion on an outlet side of the inner peripheral surface and divergingly tapers in a direction toward a downstream side of the first valve seat.