Abstract: A water quality diagnosis method, includes: a step of obtaining a measurement value of electrical conductivity of a sample water derived from a steam or a circulating water obtained from a steam turbine plant using ammonia as a water conditioner and a measurement value of pH of the sample water; and a determination step of determining presence or absence of abnormality of water quality of the steam turbine plant by using at least a first determination condition of whether the measurement value of the electrical conductivity and the measurement value of the pH are included in a first determination region which is set within a first correlation map of the electrical conductivity and the pH taking into account a carbonic acid concentration range where carbonic acid is dissolvable in the sample water.