PURPOSE: To detect mixture of oil in the early morning and to reliably remove oil by providing a means to separate oil leaking to the water side by utilizing a difference in a surface tension and sound an alarm when, after oil is collected to a bottom part through utilization of a difference in specific gravity, oil is raised to a specified level.

CONSTITUTION: Return cooling water flowing from an oil cooler 1 through a cooling water return pipe 11 flows in a water storage tank 30. In which case, the water storage tank 30 has an atmosphere air release vent 17 and is roughly divided into an oil separating device 31 and a water storage tank 32. In a precipitator 34 of the oil separating device 31, oil is separated by utilizing a difference in a surface tension by an oil separation film group 35, and collected at a bottom through a funnel 37. As a result, when oil is raised to a specified level, it is detected by an electrostatic capacity type oil level detector 45 and at the same time when a detecting signal is outputted to a remote monitoring panel 46, an alarm is sounded. Meanwhile, the lowest part of the funnel 37 is connected to a piping 38, a stop valve 39, and an overflow pipe 40, a drain valve 42 is opened and oil is recovered to a treating system 43 as confirmation is effected by a side glass 41.