A crack detection method, which comprises the steps of: taking, by a predetermined imaging unit, an image of an outermost coating surface of a predetermined part to be detected, of a detection object, which part comprises a main body formed of a conductive material, the main body is coated from outside with a coating layer formed of a non-conductive and non-magnetic elastically deformable material; applying heat to the part to be detected, the image of said coating surface of which has been taken, to increase a temperature of the part to a predetermined temperature by which said coating layer may not degrade, by an induction heating applied to the part to be detected by means of a predetermined heating unit and/or a heat transfer from a part in a vicinity of the part to be detected, by applying heat by the induction heating to the part in a vicinity of the part to be detected; taking an image of said coating surface of the part to be detected to which the heat has been applied before a temperature of the coating layer changes due to transfer of heat from the part to be detected and the coating layer itself deforms due to the temperature change, by means of said imaging unit once again; and determining, by a predetermined image analysis unit, strain by analysis from two images of the coating surface of the part to be detected, which have taken by said imaging unit before and after applying the heat, in respective positions of the part to be detected, to acquire a strain distribution in the part to be detected; and wherein: the strain distribution in the part to be detected includes, in a case where a crack exists in the part to be detected, a strain area in which strain is concentrated approximately line-like and has a different magnitude and direction form from strain of other areas around the strain area in the strain distribution, in response to a small displacement of an area with the crack due to application of the heat. taking, by a predetermined imaging unit, an image of an outermost coating surface of a predetermined part to be detected, of a detection object, which part comprises a main body formed of a conductive material, the main body is coated from outside with a coating layer formed of a non-conductive and non-magnetic elastically deformable material; applying heat to the part to be detected, the image of said coating surface of which has been taken, to increase a temperature of the part to a predetermined temperature by which said coating layer may not degrade, by an induction heating applied to the part to be detected by means of a predetermined heating unit and/or a heat transfer from a part in a vicinity of the part to be detected, by applying heat by the induction heating to the part in a vicinity of the part to be detected; taking an image of said coating surface of the part to be detected to which the heat has been applied before a temperature of the coating layer changes due to transfer of heat from the part to be detected and the coating layer itself deforms due to the temperature change, by means of said imaging unit once again; and determining, by a predetermined image analysis unit, strain by analysis from two images of the coating surface of the part to be detected, which have taken by said imaging unit before and after applying the heat, in respective positions of the part to be detected, to acquire a strain distribution in the part to be detected; and wherein: the strain distribution in the part to be detected includes, in a case where a crack exists in the part to be detected, a strain area in which strain is concentrated approximately line-like and has a different magnitude and direction form from strain of other areas around the strain area in the strain distribution, in response to a small displacement of an area with the crack due to application of the heat.