A cooling water circuit includes: a heat-radiating heat exchanger in which a cooling water is heated by heat radiated from an external heat medium; a heat-absorbing heat exchanger in which a cooling water is cooled by heat absorbed by an external heat medium; and a first heat exchanger and a second heat exchanger in which heat is exchanged between the cooling water and outside air. When a high-temperature cooling water heated by the heat-radiating heat exchanger flows through the first heat exchanger and the second heat exchanger, the first heat exchanger and the second heat exchanger are arranged in series for the cooling water to flow. When a low-temperature cooling water cooled by the heat-absorbing heat exchanger flows through the first heat exchanger and the second heat exchanger, the first heat exchanger and the second heat exchanger are arranged in parallel for the cooling water to flow.