PURPOSE: To easily measure an unburnt amount from the integration value of the temp. difference between coal ash and a reference substance obtained when the unburnt substance of coal ash is perfectly burnt by heating coal ash and the reference substance under a same condition from the relational curve of a preliminarily calculated integration value and the unburnt amount within a short time.

CONSTITUTION: In measuring an unburnt component of ash or clinker generated in a coal firing boiler or an electric furnace, a coal ash specimen 22 and a reference substance such as α- Al2O3 are respectively received in the containers 24, 25 of a measuring apparatus. The containers 24, 25 are provided to the upper end of the support pipe 21 of a standard 20 having an air and O2 gas introducing port 20a and, after the specimen and the reference substance are received, covered with a protective pipe 26 having exhaust port 26a. The temp. measuring thermocouple 28 of the specimen 22 and the temp. difference measuring thermocouples 31, 32 of the specimen 22 and the reference substance 23 are respectively inserted under the containers 24, 25. The pipe 26 is heated at a predetermined temp. rising speed from the outside by a furnace 27 and the temp. reference curve of the specimen 22 and the reference substance or the area of a heat generating peak, that is, the integration value S of the temp. difference is calculated. By this method, the unburnt component in the specimen can be measured within a short time by a simple apparatus.