Previously we have succeeded in exploiting a new high sensibility eddy current testing (ECT) sensor called the infinite (∞) coil. Operating principle of this ∞ coil is that two adjacent coils constructing the north and south poles alternatively set a zero magnetic potential region and keep the same situation when eddy currents are flowing along the paths in parallel to the exciting coils. If the eddy currents could not flow the paths in parallel to the exciting coil on the test specimen, then a zero magnetic potential region between the two exciting coils moves toward the other positions. This means that a sensing coil located at a zero magnetic potential region between the two exciting coils is possible to detect the disturbed magnetic fields, i.e., the defect in the test specimen could be detected from sensing coil signal. In this paper, we improve the ∞ coil in an optimum manner which leads to a practical factory product. Employment of the two planar semicircular exciting coils has made it possible to enlarge the zero magnetic potential region. Further a planar rectangular shape exciting coils may be considered as a deterministic design from the viewpoints of practical production.