To provide a deterioration diagnosing device capable of accurately evaluating a theoretical value and an observation value of a heat transmission rate, since the theoretical value and the observation value of the heat transmission rate cannot be accurately evaluated, due to respectively calculating the theoretical value of the heat transmission rate from the gas side and the observation value from the steam (water) side, in a conventional technology.

The heat transmission rate to steam from gas in a heat exchanger for constituting a boiler, is characterized by being set as the other heat transmission rate, heat conductivity of a pipe, and heat conductivity of steam and gas, by calculating one of the heat transmission rate on the basis of the heat conductivity of the pipe and the heat conductivity of the steam and the gas. The heat transmission rate is calculated on the basis of the rate.