This paper describes the fundamental studies on detecting delamination in walls by using thermal image. Firstly, It was made clear that probability of detecting delamination in various types of walls, and the thickness of delamination layer and mortar layer can be estimated from the time and the value of maximum temperature difference between normal portion and delaminated portion. Secondly, we proposed an image processing method for detecting delamination in walls and confirmed the effectiveness of this method from experimental results. In this method, thermal influence of pollution and crack on the surface can be eliminated by using thermal image in 5. 5〜7. 9μm wavelength region and an image of surface temperature distribution which was simulated heat balance using a digital image of visible rays.