A magnetic testing method applying a rotating magnetic field to a material to be tested and detecting a flaw based on a testing signal generated by the rotating magnetic field, the method comprising the steps of: extracting a candidate flaw signal by using an alternating current obtained by superpositioning a first current and a second current having a lower frequency than the first current, as an exciting current for exciting the rotating magnetic field, synchronously detecting the testing signal by using the first current as a reference signal, and thereafter synchronously detecting by using the second current as a reference signal; displaying a testing image, being a color image, constructed by a plurality of pixels corresponding to respective positions of the material to be tested, wherein the pixels are colored into different colors in correspondence to the phase of the candidate flaw signal and each of the pixels has an intensity corresponding to an intensity of the candidate flaw signal at each of the positions, so that the testing image allows that a phase of the candidate flaw signal at each of the positions in the testing image can be identified by the color of the pixels, or a testing image comprising a plurality of gray images each constructed by a plurality of pixels corresponding to respective positions of the material to be tested, wherein the phase or a range of the phase of the candidate flaw signal included in each of the gray images is different and each of the pixels has an intensity corresponding to an intensity of the candidate flaw signal at each of the positions, so that the testing image allows that a phase of the candidate flaw signal at each of the positions in the testing image can be identified by the plurality of the gray images; and detecting the flaw based on the displayed testing image. extracting a candidate flaw signal by using an alternating current obtained by superpositioning a first current and a second current having a lower frequency than the first current, as an exciting current for exciting the rotating magnetic field, synchronously detecting the testing signal by using the first current as a reference signal, and thereafter synchronously detecting by using the second current as a reference signal; displaying a testing image, being a color image, constructed by a plurality of pixels corresponding to respective positions of the material to be tested, wherein the pixels are colored into different colors in correspondence to the phase of the candidate flaw signal and each of the pixels has an intensity corresponding to an intensity of the candidate flaw signal at each of the positions, so that the testing image allows that a phase of the candidate flaw signal at each of the positions in the testing image can be identified by the color of the pixels, or a testing image comprising a plurality of gray images each constructed by a plurality of pixels corresponding to respective positions of the material to be tested, wherein the phase or a range of the phase of the candidate flaw signal included in each of the gray images is different and each of the pixels has an intensity corresponding to an intensity of the candidate flaw signal at each of the positions, so that the testing image allows that a phase of the candidate flaw signal at each of the positions in the testing image can be identified by the plurality of the gray images; and a testing image, being a color image, constructed by a plurality of pixels corresponding to respective positions of the material to be tested, wherein the pixels are colored into different colors in correspondence to the phase of the candidate flaw signal and each of the pixels has an intensity corresponding to an intensity of the candidate flaw signal at each of the positions, so that the testing image allows that a phase of the candidate flaw signal at each of the positions in the testing image can be identified by the color of the pixels, or a testing image comprising a plurality of gray images each constructed by a plurality of pixels corresponding to respective positions of the material to be tested, wherein the phase or a range of the phase of the candidate flaw signal included in each of the gray images is different and each of the pixels has an intensity corresponding to an intensity of the candidate flaw signal at each of the positions, so that the testing image allows that a phase of the candidate flaw signal at each of the positions in the testing image can be identified by the plurality of the gray images; detecting the flaw based on the displayed testing image.