PURPOSE: To discover oil spillage as early as possible by measuring the difference in the oil level when it occurs in one or two phases out of three phases of an OF cable for an oil supply tank.

CONSTITUTION: Three guide pipes 10 each made of a non-magnetic material are arranged in a shape of a rough regular triangle corresponding to a single-core OF cable and main magnets 11 are incorporated thereinto respectively. The three guide pipes 10 are grouped to be passed through a hollow floating body 12. Support based 15 are fixed at respective apexes of the floating body 12 and mercury switches 16 are fastened thereon respectively in the central symmetry. When the mercury switches 16 are so equal in the liquid level as to keep the floating body 12 horizontal, electrodes are opened therebetween and closed with the tilting of the floating body. The guide pipes 10 are communicated separately with an oil supply tank through a bypass piping from the top and bottom thereof. When an oil spillage occurs in one or two phases of an OF cable or in the oil supply tank, a difference in the liquid level develops and the mercury switches are closed to sound an alarm.