PURPOSE: To support sure maintenance by reporting the overall condition of a main machine at any time to detect a fault.

CONSTITUTION: A ship main engine plant consisting or the main engine, a propulsion auxiliary machine, etc., is provided with an arithmetic unit 1, a data input device 3, a storage device 2, and a display device 5, and not only data from instruments of the main engine 4 can be directly inputted to the input device 3 but also data picked up on the main engine side and data acquired from the outside can be inputted there, and operation formulas, in accordance with which a data base for diagnosis, performance data, and data inputted from the input device 3 are used to calculate performance information and combustion information of the main engine 4, and a data base for maintenance management are stored in the storage device 2, and performance information and combustion information of the main engine 4, information of items required for search of the cause of a fault, and information of necessities of preparation of principal parts required for maintenance management are calculated by the arithmetic unit 1 and are selected by the operation from the outside and can be displayed on the display device 5, and abnormality estimated pauses and fault avoiding means or estimated causes and estimated preparation necessary positions are displayed in the descending order of priority.