To establish a technique of measuring the distribution of heat load at the optional point in a furnace at optional time.

By using a portable furnace combustion-diagnosing device provided with a probe 2 fitted with a light receiving part 1 measuring the quantity of the incident radiation from the inside of the furnace, a probe cooling medium channel provided in the probe 2, a device for circulating cooling medium connected with the probe cooling medium channel and purge gas supply device generating the gas purging the inside of the light receiving part, the probe 2 is detachably fitted to an observation port on the wall surface of the furnace through a heat-resistant insertion seat, and the distribution of heat load received by the furnace wall at the time of operating the furnace is measured. An atmospheric gas temperature distribution is calculated from the obtained heat load distribution, and the combustion diagnosis in the furnace predicting the condition of ash adhesion in the furnace is performed.