A cooling system (1A) is installed in a cooling circuit (100A) which is provided with a water pump (W/P) (1) that circulates a cooling liquid for an engine (2), and a radiator (6) that cools the cooling liquid for the engine (2). The cooling system (1A) is provided with: a first path section (11A), which is provided between the engine (2) and a cooling liquid outlet of the W/P (1),a second path section (12A), which is provided between the radiator (6) and the cooling liquid inlet of the W/P (1),and a rotary valve body (13), which is provided between the path sections (11A, 12A), and is capable of simultaneously controlling circulation of the cooling liquid in the first path section (11A) and circulation of the cooling liquid in the second path section (12A) by rotary operation.