In an internal combustion engine, a thermostat valve for changing over between coolant circulation through a radiator-routing passage to coolant circulation through a bypass passage, is provided with a first valve for opening and closing the radiator-routing passage, and a second valve for opening and closing the bypass passage. The first and second valves and are operable concurrently. A cylinder coolant jacket around cylinder bores of a cylinder portion is partitioned into two in a cylinder axis direction to thereby form a main cylinder coolant jacket on a side of a cylinder head portion and a sub-cylinder coolant jacket on a side of a crankcase portion. The bypass passage is formed partly by the sub-cylinder coolant jacket. The above arrangement expedites warming-up during the engine start and achieves favorable appearance by a simplified structure.