The effects of the ECT (Eddy Current Test) Signal influenced by TSP (Tube Support Plate) blockage by sludge pile at nuclear power plant SG (Steam Generator) are studied in this paper. Model 51B SG has quatrefoil type flow holes in TSP and has a tendency to clogging up the flow holes at the flow-stagnated region with the sludge. The sludge pile deposited on the TSP flow holes makes the flow be blocked, and causes the steam generator water level oscillation. Therefore, it is necessary to know how much portion of the TSP holes is obstructed by the sludge pile for the safe operation. Some methods are used to measure the extent; visual test (VT), wide range water level monitoring, and ECT. The method introduced in this paper is based on the ECT. The TSP blockage is simulated in a mock-up with the artificially fabricated sludge pile. The relationship between the extent of blockage and the ECT signal acquired by the bobbin and MRPC (Motorized Rotating Pancake Coil) is studied. A quantitative result for the blockage is obtained. A conversion table is developed from the MRPC probe test, which helps to estimate the blockage level from the bobbin probe signal. Applying the technique introduced in this paper to S/G tube inspection with other method complementarily, it is expected to improve the accuracy of estimating the flow hole blockage level and to give a guidance to SG management program.