An engine cooling system controls a cooling degree of an engine 1 according to an operating condition thereof by circulating cooling water through the engine. This system is provided with a water temperature sensor 31 for detecting a cooling water temperature THW1, a flow rate regulating valve 8 for regulating a circulation flow rate of the cooling water, and an electronic control unit (ECU) 30 for controlling the valve 8. The ECU 30 sets a non-control region and a first and second control regions, each centering on a target water temperature to be calculated according to operating conditions of the engine 1. The ECU 30 controls the valve 8 to open and close at a high speed when the cooling water temperature THW1 is in the second control region, thereby to allow the cooling water temperature to approach the first control region or at a low speed when the cooling water temperature THW1 is in the first control region, thereby to allow the cooling water temperature to approach the non-control region, or holds the valve 8 at a current opening degree when the cooling water temperature THW1 is in the non-control region.