A cooling system for an internal combustion engine in which bubbles are not transferred to the water pump, and thereby the efficiency of the water pump is maintained. The interior of a water outlet mounted on a cylinder head is divided by a partition wall to form a feeding chamber to feed the coolant introduced from the cylinder head to a first path provided with a radiator, and a receiving chamber forming a second path to receive the coolant returned from a heater core. Between the receiving chamber and the feeding chamber is a channel making one or both chambers communicate with the other. As a result, bubbles in the coolant running through the second path are introduced into the feeding chamber through the channel and removed from the radiator.