The value of a variable reflecting the operating condition of mechanical equipment is continuously detected as operation condition data. A long period average value of the operating condition data detected in long reference period, and a short period average value of data detected in a short analysis period ending in the present time are calculated. Just how much the short period average value differs from the long period average value is adopted as an index of abnormality and using this index, it is possible to ascertain abnormal operating conditions regardless of the absolute value of the variable detected. The above short period average value, long period average value and standard deviation thereof are applied to a first fuzzy membership function for calculating the degree of stability of the mechanical equipment therefrom. Furthermore, the degree of stability is then applied to a second fuzzy membership function for calculating the amount by which the operating parameter of a control factor of the mechanical equipment must be adjusted in order to achieve a desired degree of stability. Thus the mechanical equipment can be controlled for optimum operation.