PURPOSE: To measure a current in a connecting lead without being affected by an electric field and a magnetic field, by providing an earthing cylindrical electrode between the outer-peripheral part of a connecting lead wire and the inner- peripheral surface of a connecting duct, and by fitting a sensor consisting of an optical fiber having a magnetooptical effect, etc. on the outer periphery of said electrode.

CONSTITUTION: A high-voltage connecting lead wire 8 is led out from a winding 2 housed in a tank 3 and connectd to a high-voltage bushing through a duct 4. This lead wire 8 is covered with a lead insulator 17, while an earthing cylindrical electrode 19 is provided at the outer-peripheral part of the lead insulator 17 located inside the duct, through the intermediary of a supporting insulator 18. An optical fiber 20 having a magnetooptical effect is wound round the outer periphery of the electrode 19, and an optical fiber 21 is connected, as a lead for the input and output of a signal, to the optical fiber 20. By measuring a current in the optical fiber 20, a current in the winding of a transformer can be measured under no adverse effect of an electric field and a magnetic field.