1. A crack detection system, which comprises: an imaging unit that takes 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 an conductive material and a coating layer formed of a non-conductive material, with which the main body is coated from outside; a heating unit that applies heat to at least surface area of said part to be detected, to increase a temperature of said coating layer to a predetermined temperature by which said coating layer may not degrade, by an induction heating applied to the part to be detected and/or a heat transfer in a vicinity of the part to be detected, along with the induction heating applied to the part to be detected; and an image analysis unit that analyzes two images of the same detection object, which have been taken at different times, to determine strain between the two images in respective positions within a range for detection and acquire a strain distribution within the range for detection; wherein: said imaging unit takes the image of said coating surface of said part to be detected before applying the heat by means of said heating unit and immediately after applying the heat; said image analysis unit determines the strain by the analysis from the two images 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 the strain distribution in the part to be detected; and the strain distribution in the part to be detected includes, in case where a crack exists in the part to be detected, a group of strains in which strains are concentrated almost linearly in a quite different form from other strains around the group of strains in the strain distribution, in response to a small displacement of an area with the crack due to application of the heat.