PURPOSE: To provide the expert system knowledge acquiring system which can define relation between the cause of a fault and the condition even without the case of the fault concerning another device.

CONSTITUTION: This system is equipped with a device hierarchy managing means 7 to manage the hierarchy of device groups, knowledge base 12 to store the hierarchical data of the device groups, the condition data of the fault and fault cause data, retrieving means 3 and 4 to retrieve the knowledge base 12 based on input information, additional registering means 5 and 6 to additionally register the condition of the device and cause data when the condition of the device and the cause data inputted to the knowledge base 12 are not registered, condition/fault cause relation managing means 8 to acquire and manage the relation between the fault cause and the condition, and knowledge propagating means 9 to propagate the knowledge concerning the relation between the fault cause and the condition in the abovementioned hierarchy.