1. A method of assessing the life of a member subjected to a high in-service temperature for a long period comprising the steps of: determining a Larson-Miller parameter for the member whose life is to be assessed from the in-service temperature and a service time period during which the member is used in-service temperature, and calculating the creep damage degree on the basis of cumulative damage rules from the hardness and stress of the member to establish data; and approximating the relationship between the Larson-Miller parameter and the creep damage degree by an expression including an exponential function.