PURPOSE: To prevent the titled electrode unit against corrosion by a method wherein the external electrode unit applied to a boiler or the like, a packing is fitted into an electrode rod inset hole provided in the body of the boiler so as to cover the open end of the hole and an electrode rod is inserted into the packing.

CONSTITUTION: The electrode rod insert hole 13 is formed in the body 12 of the boiler coated with an anticorrosive film 11, a socket 16 is fixed to the hole and the packing 14 is fitted in, and fixed to, the hole 13 by means of a stop metal 17 screwed into the socket 16. Then the electrode rod 15 is inserted into a hole 17a in the stop metal 17 and a hole 14a in the packing 14 so that the top end of the electrode rod 15 projectes into the body of the boiler. With the above structure, the ground metal of the body 12 of the boiler around the electrode 15 is not exposed and as a result, there arises no possibility of a large amount of electric current flowing through the exposed ground metal causing the deterioration of the anticorrosive property of the boiler or it is possible to eliminate completely the disadvantage that calcium carbonate adheres to the electrode rod.