PURPOSE: To improve rise of heating and stability after the rise, by a method wherein the one of 2 hot water outlet ports is positioned in the vicinity of a position under a liquid surface in a water jacket and the other is positioned below the one port, and the outlet ports are selectively opened and closed depending on the boiling condition of coolant.

CONSTITUTION: During heating-up operation after the starting of an engine 1, a water jacket 2 is filled with cooling liquid. In which case, a control circuit 11 discriminates heating-up operation by means of signals from liquid surface sensors 10 and 20 and a temperature sensor 14, and a first electromagnetic valve 34 is opened and a second electromagnetic valve 35 is closed. Thus, when a circulating pump 27 is driven along with operation of a heater, cooling liquid having a relatively high temperature is sucked out through an upper first hot water outlet 30 and is fed to a heater core 25. Meanwhile, when the liquid surface of the cooling liquid is lowered to a specified value and a liquid temperature is increased to higher than a given value, the control circuit 11 discriminates it to close the first electromagnetic valve 34 and open the second electromagnetic valve 35. This causes hot water containing less steam to be guided out through a second hot water outlet 31.