The high sensitivity of magnetic tunnel junctions (MTJs) is independent of the field frequency, and sensors based on them have shown considerable potential in measuring secondary fields during eddy current nondestructive testing (ECT). In this study, sensors with 4, 16, 28, 40, and 52 serial MTJs were fabricated and characterized for detecting variations in magnetic fields. These micrometer-sized TMR sensors with serial MTJs exhibited low detectivities (2.0 ∼ 5.1 nT/Hz0.5 at 100 Hz). Furthermore, in ECT measurements, the sensors were used to detect far-side pits in a copper specimen, and the sensor with 28 serial MTJs offered the highest signal-to-noise ratios when detecting far-side pits with various diameters (2.5, 5.0, and 7.5 mm). In addition, the TMR sensors could detect millimeter-sized far-side pits having various depths and diameters, suggesting that they could detect buried defects in ECT.