The development and applications of the tester detecting ferromagnetic objects is described. The tester can be used to detect and evaluate quantitatively blockages by rust in superheater tubes and reheater tubes so as to avoid explosion of the tubes. By using the magnetic measurement method,five points where blockage detection will be made are set on tube surface along a circumference. Simulations by finite element method and calculation of calibration curves are conducted. The results of calculation are verified by experiments. Comparing with the calculated values, the standard error of the measured values, which is named as the relative error, is 5.2%. The standard error of the field-measured proportion of blocked area in the tube to the area of the cross section of the tube bore is 3.9%, compared with that measured by X-ray photography, which is named as absolute error. Blockage detections on tubes have been implemented for the five power plants. More than 30 blockages the area percentage of which is more than 30% have been found. Cutting the tubes and removing the blockages out of the tubes indicates that no false report of blockage have happened. It is concluded that blockage detection on the tube using the tester is effective for prevention of the tube from explosion accident.