Eddy Current Testing Method, Steel Pipe or Tube Tested by the Eddy Current Testing Method, and Eddy Current Testing Apparatus for Carrying out the Eddy Current Testing Method

An eddy current testing apparatus comprising:

• a differential coil disposed to be opposite to a metal material having magnetism, for inducing an eddy current by making an AC magnetic field act on the metal material, and detecting the eddy current induced in the metal material, and
• a signal processor for energizing the differential coil with an AC current, and detecting a local high hardness part occurring in the metal material based on a detection signal outputted from the differential coil,
• wherein the signal processor sets a frequency of the AC current for energizing the differential coil so that a phase difference between a magnetism variation signal of the metal material and a liftoff signal detected by the differential coil is equal to or larger than 135°, and detects a local high hardness part occurring in the metal material based on an amplitude and a phase of a detection signal outputted from the differential coil.