A motorcycle comprising a cooling system structure and a water-cooled internal combustion engine, comprising: a cylinder section constituting, together with a crankcase for rotatably supporting a crankshaft, an engine main unit, said cylinder section includes a cylinder body that projects upwardly from the crankcase, a piston operatively connected to the crankshaft is slidably fitted within the cylinder body and a cylinder head is connected to the cylinder body; water jackets are formed in the cylinder body and the cylinder head, respectively; a radiator is disposed forwardly of the engine main unit in a longitudinal direction of the motorcycle; a water pump is disposed on the engine main unit, the water pump (54, 154) circulates coolant through the radiator and the water jackets; said water jackets are respectively formed so as to allow coolant to flow through the cylinder body and the cylinder head independently of each other; and a thermostat is disposed on a front wall surface of the cylinder section in the longitudinal direction of the motorcycle, the thermostat being capable of selecting a condition in which the water jacket in the cylinder body and the water jacket in the cylinder head directly communicate with each other or a condition in which the water jacket in the cylinder body and the water jacket in the cylinder head communicate with each other via the radiator, characterized in that the thermostat is disposed at a central portion of the cylinder section in a direction extending along an axis of the crankshaft, wherein the length of a line that extracts coolant from the thermostat to the radiator is equal to that of a line that returns the coolant from the radiator to the thermostat. a cylinder section constituting, together with a crankcase for rotatably supporting a crankshaft, an engine main unit, said cylinder section includes a cylinder body that projects upwardly from the crankcase, a piston operatively connected to the crankshaft is slidably fitted within the cylinder body and a cylinder head is connected to the cylinder body; water jackets are formed in the cylinder body and the cylinder head, respectively; a radiator is disposed forwardly of the engine main unit in a longitudinal direction of the motorcycle; a water pump is disposed on the engine main unit, the water pump (54, 154) circulates coolant through the radiator and the water jackets; said water jackets are respectively formed so as to allow coolant to flow through the cylinder body and the cylinder head independently of each other; and a thermostat is disposed on a front wall surface of the cylinder section in the longitudinal direction of the motorcycle, the thermostat being capable of selecting a condition in which the water jacket in the cylinder body and the water jacket in the cylinder head directly communicate with each other or a condition in which the water jacket in the cylinder body and the water jacket in the cylinder head communicate with each other via the radiator, characterized in that the thermostat is disposed at a central portion of the cylinder section in a direction extending along an axis of the crankshaft, wherein the length of a line that extracts coolant from the thermostat to the radiator is equal to that of a line that returns the coolant from the radiator to the thermostat.