PURPOSE: To support identification of a failure origin and to judge the failure origin within a region of each board by providing a knowledge storing mean of object system constitution, a knowledge storing mean of diagnosing procedure, a preparation mean of diagnosing procedure, a selection mean of probable failure origins, a judgement mean of failure origin and so on.

CONSTITUTION: At a knowledge storing mean 1 of object system constitution, constitutional knowledge consisting of knowledge about the object system constitution (names, connecting relationship and so on of circuits constituting a system controlling system), which enables identification of failure origins separately for each system, is stored separately. A knowledge storing mean 2 of diagnosing procedure stores knowledge regarding diagnosing procedure peculiar to the object and consisting of inspection condition such as inspection procedure, separately for each system and for each failure identification unit. The stored contents of both means 1 and 2 are fed to a preparation mean 4 of diagnosing procedure, along with input informations from an input mean 3 feeding informations of current situation of the system, and the like. A selection mean 5 of probable failure origins selects the probable failure origins for each identification unit of failure origin, and the they are stored in a storing mean 6 of candidate failure origins, with relation to a situation pattern corresponding to inspection signals. A judgement mean 7 of failure origin judges whether the situation pattern of inspection signals is stored in the mean 6 or not, when the pattern from the mean 3 is fed.