This paper proposes a method of measurement of the thermal resistance of scale adhered to the inner surface of a boiler tube. The principle of the method is to measure the temperature rise due to the scale when the tube is radiatively heated from outside. An experiment was performed with a test piece composed of three tubes of 30 cm in length, which had been cut away from a supercritical pressure boiler. 120 mm×140 mm area of the test pieces was radiatively heated, boiling of water took place inside the tubes, and the temperature at the top of the tube was measured with welded thermocouples of 80 μm, in diameter. Similar measurement was performed after cleaning the inside of the tubes with hydrochloric acid, and both measured data were compared. An example of the results is as follows;the difference of the temperature rise was 8.24 K for heat flux of 100 kW/m2, and the estimated thermal resistance of the scale was 0.082 m2 K/kW.