An engine cooling system (2) includes a first passage (11) communicating a radiator (8) with a jacket inlet (7) of a water jacket of an engine, a second passage (13) communicating a jacket outlet (12) of the water jacket with the radiator, and a third passage (16) communicating a part of the second passage with a part of the first passage located between the radiator and the water pump. To allow the cooling water temperature circulating in a water jacket of an engine to be detected accurately, a water temperature sensor (72) is provided in a part of the second passage intermediate between the part of the second passage where the third passage branches off and the jacket outlet.