PURPOSE: To detect the mechanical deformation and the initial stage of the partial overheat by winding a conductor which is covered by a solid state insulating material and passing at least one piece of optical fiber between the conductor and the solid state insulating material or in the solid state insulating material.

CONSTITUTION: The optical fiber 12 is passed between the conductor comprising copper, aluminum, or the like and the covering solid state insulating material 11 such as paper, plastic film, and the like, thereby the winding is constituted and used in a transformer. If the winding is normal , the light from a light emitting device 14 passes the optical fiber 12a, enters into the winding, propagates through the optical fiber 12 along the conductor 10, and passes the optical fiber 12a. The light is detected by a light receiving device 15 and indicated by an indicator 16. If the transformer is short-circuited, the winding is deformed, or broken in the severe case. The optical fiber is broken at the initial stage of the short-circuit of the winding, the light receiving device 15 does not detect the light, and abnormality can be externally detected.