In a warm-up system that pre-warms up of an engine (E) by transferring heat from a heat source to an engine cooling water circulation circuit (1) of a vehicle, a residential hot water circuit (2) that uses a household heat source is provided as the heat source, and a one-touch connector (3) is provided in the engine cooling water circulation circuit (1) and the residential hot water circuit (2). The one-touch connector (3) connects the two circuits (1, 2) prior to start-up of the engine (E) such that heat is transferred to the engine cooling water circulation circuit (1), and disconnects the two circuits (1, 2) following heat transfer in preparation for start-up of the engine (E).