An equipment is diagnosed by monitoring a career of operations of the equipment necessary for diagnosing the equipment and a condition of the equipment during the operation, monitoring information on equipment operations and information on variations in operation of the equipment having started operation for a data gathering purpose, selecting some of the gathered data, classifying the selected data based on rules, computing characteristics of variations in equipment state during the operation of the equipment and diagnosing the data on the basis of criterion references for the characteristics prescribed by the rules as an equipment diagnostic procedure.