PURPOSE: To provide a diagnosing device for cause of abnormalities in boiler by which the cause of abnormalities can be diagnosed without entailing a large burden such as memory expansion to the microcomputer on the side of a boiler and assembly of a knowledge base in the computer.

CONSTITUTION: In a boiler which carries out control of operation by connecting a microcomputer on the side of the boiler and computer on the side of a control station, contents of abnormalities and data on the conditions of various devices that are recorded in an abnormality content judgement program 12 of the compute 11 on the boiler side are sent to the computer 21 on the control station side. Based on the knowldge base 22 of diagnosing cause of abnormalities in the computer 11 on the boiler side inference of cause of abnormalities is made by an inference engine 23, and the results of this inference are sent to the microcomputer 11 on the boiler side, and the inferred caused of an abnormality is displayed on a display device 16.