To provide a method and an apparatus for conveniently, quickly, quantitatively and accurately measuring and diagnosing the corrosion resistance of a water pipe provided in a heat exchanger.

In the method and apparatus for measuring an electrical resistance of the water pipe 93 provided in the heat exchanger, a first terminal 97 is made to contact the inner face of the water pipe 93, a second terminal 98 is separated from the first terminal 97 and is made to contact the water pipe 93, a current is carried between both terminals, and the electrical resistance is measured. In particular, the first terminal 97 can be deformed elastically and the terminal 97 is inserted into the water pipe 93, and preferably, elastically contacts the inner face of the water pipe 93 by the elasticity of the terminal 97.