The IR camera (10) takes an IR thermal image of a surface of the structure (40). In the IR thermal image, temperature gradient is superposed besides temperature difference between non-defective and defective regions of the structure. The image processing unit (21) of the analysis unit (20) produces an image indicating distribution of a temperature variation other than a temperature gradient by extracting the distribution of the temperature variation from the IR thermal image. The image display unit (30) displays the image produced by the image processing unit (21). Since the distribution of the temperature variation other than the temperature gradient is extracted from the IR thermal image, a temperature difference between defective and non-defective regions in the structure (40) can be clearly displayed. Therefore, even if there exists a temperature gradient on the structure surface, the defect location in the structure can be easily determined.