A coolant circulation circuit of an internal combustion engine includes a heat accumulation tank having a heat insulating structure and a heater core for heating air for heating a compartment in the vehicle. High temperature coolant is reserved in the heat accumulation tank even when the engine is stopped. An intake air heat exchanger is disposed in the circuit on the downstream side of the heater core, and high temperature coolant can effectively heat the intake air. An engine lubricating oil heat exchanger and a transmission operating oil heat exchanger are arranged in the circuit on the downstream side of the intake air heat exchanger. Intake air, engine lubricating oil and transmission operating oils are suitably heated during warm-up of the engine to thereby improve fuel economy and reduce exhaust gas pollution.