In this study, we developed and characterised a method for detecting microcracks in steel materials by eddy current testing (ECT). Magnetic saturation ECT is applied because magnetic noise is generated by steel materials during ECT. Although direct-current (DC) magnetisation is generally used for magnetic saturation ECT, the use of alternate-current (AC) magnetisation offers several advantages. When using AC magnetisation for surface flaw detection, the magnetisation is not influenced by the thickness of the test object owing to the skin effect; therefore, demagnetisation is unnecessary. In this study, we evaluated microcrack detection results obtained using AC magnetisation ECT with different synchronous positions (conditions) of the magnetisation and ECT; and identified the optimum synchronous position. In addition, the magnetisation distribution in a test object was evaluated by the finite element method analysis and the flaw detection results were verified.