In a cooling system structure for a vehicular water-cooled internal combustion engine with a cylinder section and a cylinder body and a cylinder head. The cooling system structure having water jackets formed in the cylinder body and the cylinder head, respectively, with a radiator disposed forwardly of the engine main unit. A water pump performs forced circulation of coolant through the radiator and the water jackets, to permit routing of pipes with enhanced appearance. Water jackets are formed, respectively, in a cylinder body and a cylinder head such that coolant flows through the cylinder body and the cylinder head independently of each other. A thermostat is mounted on a front wall surface of a cylinder section and is capable of selecting a condition in which the water jackets directly communicate with each other or a condition in which the water jackets communicate with each other via a radiator.