An internal combustion engine is provided with a heat-generation chamber disposed so as to be adjacent to a combustion chamber, a heat-generation chamber valve capable of setting up either a communicating state or a shut-down state between the heat-generation chamber and the combustion chamber, and a heat-recovery pipe for recovering heat of a gas guided into the combustion chamber and utilizing the heat as recovered for warm-up. The heat-generation chamber is provided with a heat-generation-chamber spark plug. A catalytic unit is provided between the heat-generation-chamber spark plug and the heat-recovery pipe.