1. A non-destructive testing system using a non-destructive testing method by infrared lock-in thermography, wherein the non-destructive testing method: performs heating of a surface on one side of a lamination joining portion of different metal materials from a heating light irradiation source, the heating being in a certain preset waveform; obtains infrared images from radiant energy from the surface on the one side, the radiant energy being caused by the heating in the certain waveform, the infrared images being obtained with a certain cycle by an infrared camera; obtains an amount and a phase of infrared light by computation processing, based on brightness of each unit pixel of the obtained infrared images; and determines whether joining of the different metal materials is effective or defective, the non-destructive testing system comprising: a testing object handling device that transports and grips a testing object; a testing position control unit that controls the testing object handling device and thereby sets the surface of the lamination joining portion of the testing object to certain positions and certain directions respectively relative to the heating light irradiation source and the infrared camera; a heating control unit that controls irradiation with heating light from the heating light irradiation source; an infrared image obtaining unit that obtains the infrared images obtained by the infrared camera, based on heating in the certain preset waveform from the heating control unit; a phase image obtaining unit that obtains an amount and a phase of infrared light by computation processing, based on brightness of each unit pixel of the obtained infrared images, and further creates a phase image; and an effectiveness/defectiveness determining unit that, based on the obtained phase image, determines whether or not a region with a phase delay larger than a certain preset determination value is present in the lamination joining portion, wherein when infrared intensity of an infrared image obtained in an initial period of the heating out of the infrared images obtained, based on the heating in the certain preset waveform, by the infrared image obtaining unit via the infrared camera exceeds a predetermined intensity threshold, the testing position control unit adjusts the direction of the lamination joining portion while maintaining the certain positions of the surface of the lamination joining portion respectively relative to the heating light irradiation source and the infrared camera, and thereafter, the heating control unit again performs irradiation with the heating light so that the infrared image obtaining unit again obtains infrared images; wherein the region with a phase delay is a representation of phase delays of the respective pixels of the infrared images; and wherein the phase delay is a numeric value determined by performing computation processing for analysis of the phase of infrared light with respect to a temporal transition of the brightness of each pixel of the infrared images for a specific frequency.