To provide a mounting structure of the reference electrode in the heat exchanger of shell-and-tube type which can be arranged close to a tube end or a tube plate without damaging a water chamber wall, or without disturbing the water chamber cooling water flow.

A reference electrode 8 is supported by a tip of a stay 11 which is erected on a support part 10 which is inserted and embedded in an end of an arbitrary cooling pipe 2, and projected outward of the tube so as to be mounted close to a tube end of the cooling pipe 2 of the reference electrode 8 and a tube plate 4, and a lead 9 for a terminal of a potential measuring instrument to be extended from the reference electrode 8 is arranged along the surface of the tube plate 4 in a covered and protected condition, and led outside a water chamber.