PURPOSE: To provide a diagnosing device capable of easily generating a diagnostic knowledge base and easily confirming the content of the diagnostic knowledge base.

CONSTITUTION: A diagnostic knowledge base input processing part 11 generates the diagnostic knowledge base 2 directly from the error cause of diagnosis target equipment, hardware configuration, and the image drawing of faulty tree information representing a diagnostic procedure. Also, a screen display data conversion processing part 7 converts the diagnostic knowledge base 2 into the display data of a faulty tree information image, and outputs it to a display device 10 and a printer 6. Thereby, the content of the diagnostic knowledge base 2 can be inputted as being the image of a faulty tree. Also, the content of the diagnostic knowledge base 2 can be outputted to the printer 6, etc., as being the image of the faulty tree.