PURPOSE:To prevent a combustion chamber wall from an abnormal rise in temperature due to bubbles produced in a refrigerant, by driving at least either a condenser cooling fan or a refrigerant feed pump by compulsion when temperature in the combustion chamber wall goes up beyond the specified value, in case of an ebullition cooling type engine. CONSTITUTION:The refrigerant vaporized at a water jacket 22 is led into a condenser 23 via a vapor phase chamber wall 22a and a steam passage 27, cooled by a cooling fan 26 and condensed. Then, the condensed refrigerant is put back to the water jacket 22 via a lower tank 24 and a feed pump 25. Each of detection values of liquid level sensors 31 and 39, a liquid temperature sensor 32 and combustin chamber wall temperature 51 are all inputted into a control circuit 50, and when the detection value of the combustion chamber wall temperature 51 exceeds the specified value (for example, 140 deg.C) as well as the temperature rise ratio obtained in differentiating the detection temperature exceeds the setting value (for example, 80 deg.C/sec), the cooling fan 26 and the feed pump 25 are compulsorily driven for the prescribed time long.