A computer-implemented method and system for diagnosing and system for diagnosing and analyzing fault information of a product is carried out by (a) creating a fault tree representing causal relations between faults and causes thereof base on information of past faults and information concerning the structure and characteristics of the product, and storing the fault tree in a storage unit, the fault tree having branches allocated with weighting coefficients; (b) inputting new fault information of the product into the computer; (c) searching the fault tree in accordance with the weighting coefficients based on the fault information stored in the storage unit to thereby determine the cause of the fault; (d) generating and outputting information concerning an adjustment or repair of the product suffering from the fault based on the determined cause of the fault as well as the information concerning the structure and the characteristics of the product; (e) supplying information concerning the timing of the occurrence of the fault, symptoms appearing in the fault, the cause of the fault and the adjustment and repair data to a host computer through a data collecting station to thereby construct a database for the fault information; and (f) the quality of the product based on all or a part of information of the database.