PURPOSE: To monitor leakage current running in accordance with the conductivity of cooling water for safe operation of SCR, by providing a grid-shaped metal electrode in a cooling pipe of SCR and by making it contact with the cooling water.

CONSTITUTION: Electrodes 8 are provided in the upper and lower parts of a metallic connection pipe 10 connecting an insulation pipe 6 and a circulation pipe 9 made of vinyl chloride with each other and are fitted by means of packings 11 and fastening bolts 12. An ammeter 14 is connected 13 to the electrodes 8. The electrodes 8 are grid-shaped, while the connection pipe 10 is insulated a grounded board 7 by a gap G. By this constitution, when metal ions flowing out from a metallic cooling pipe in SCR sticks to the inside surface of the insulation pipe, the deterioration of insulation can be detected easily from leakage current, while the uncleanness of the cooling water and the mixing-in of alien substances can be detected from the change in the leakage current caused in accordance with that of the conductivity of the cooling water itself, whereby the device can be operated with high reliability.