PURPOSE: To prevent local corrosion caused by a sensor by providing a branch passage in parallel to the cooling liquid circulating passage of an engine outer side, and arranging ion remover, an iron concentration sensor, and a shutout valve for closing the branch passage at the time of at least engine stopping, in the branch passage from its upstream side in order.

CONSTITUTION: In the outside of an engine, a branch passage 26 is provided in parallel to an outside circulating passages 22 to 25 for cooling liquid of the engine, and all of them are connected to a water pump. An ion remover 27 for eliminating ion in cooling liquid, an ion concentration sensor 28 for making its output according to ion concentration, and a shutout calve 29 for closing the branch passage 26 at the time of at least engine stop are arranged in order in the branch passage 26 from its upstream side toward its downstream side. Since the ion concentration sensor 28 is arranged in the branch passage 26 for cooling liquid on the engine outer side, it is thus possible to prevent electrocorrosion on a cylinder block, a cylinder head, and the like caused by influence of the electrode of the ion concentration sensor 28. And also it is possible to prevent high temperature deterioration of the ion remover 27 by closing the shutout valve 29 when a cooling liquid temperature is not in a prescribed temperature range.