PURPOSE: To early detect an oil leak accident, by emitting an alarm on the basis of the detection of an oil level difference of an oil supply tank of each of the three phases of oil-filled cable.

CONSTITUTION: Three guide pipes 10 made of non-magnetic material are arranged virtually in the shape of a equilateral triangle corresponding to single- core OF cables, and a main magnet 11 is incorporated in each pipe. The three guide pipes 10 are inserted through a floating body 12 having a hollow part shaped virtually in an equilateral triangle. Around the hollow part 13, a number of magnets 14 are arranged in the direction in which they are repellent to the main magnet 11. When the floating body 12 lowers, a movable supporting table 15 contacting therewith is fixed, and further a mercury switch 16 is fixed thereon. If an oil leak occurs in the OF cables of one or two phases or oil supply tanks, a level difference is caused, and thereby the floating body 12 is inclined. When the repellent force of any one of the main magnets 11 is decreased due to the lowering of the level, the floating body 12 drops. Then the mercury switch 16 is inclined by the weight of the floating body, and thereby an alarming circuit is connected to emit an alarm.