A diagnostic device detects a fault and estimates its cause based on the degree of change of measured data attributes. A diagnostic object change degree pattern (CDP) generation unit calculates the degree of change of each attribute of data, including attributes of the object being diagnosed measured from the object being diagnosed to generate a diagnostic object CDP which is a combination of values of the degree of change of the respective attributes. A criterion CDP memory holds in store a criterion CDP, formed of a pattern of values of the degree of change of the attributes of data measured from the object being diagnosed, in association with event(s)-to-be-diagnosed on the fault sort basis or on the fault cause basis. A CDP diagnosis unit effects pattern matching between the diagnostic object CDP and the criterion CDP in the criterion CDP memory to diagnose the object being diagnosed.