To prevent the damage caused by electric leak from happening by always monitoring abnormal insulation, such as deterioration and breakage of an insulating material.

The insulation monitoring instrument 36 is provided with an electric source circuit 40 for monitoring, impressing the low monitoring voltage V0 without danger to a human body and a first monitoring circuit 41 for monitoring the abnormal insulation of the first insulating materials 30a, 30b at a first monitoring portion 37 between the first insulating materials 30a, 30b in a first supporting arm 29 for supporting a positive electrode 19. In the case of developing the abnormal insulation in the first insulating material 30a at the electrode side, a volt-meter 51 in the first monitoring circuit 41 detects the detected voltage V1 at higher than the monitoring voltage V0. Further, in the case of developing the abnormal insulation in the first insulating material 30b at the instrument body side, the volt-meter 51 in the first monitoring circuit 41 detects the detected voltage V1 at lower than the monitoring voltage V0. Then, in the case of being the detected voltage V1 at higher or lower than the monitoring voltage V0, an abnormal signal setting device 52 outputs an abnormal signal.