Diagnosing the reinforced concrete structures by using infrared thermography has become considerable in Japan. Infrared thermography has potential for detecting the defects, but is challenging due to unwanted thermal signals caused by surface roughness dirt, etc. To prevent from misdiagnosing, an innovative concept of merged thermal with visible images is proposed. Several key issues exist to capture the merged images: (1) alignment for the optical axes of an infra-red camera and digital still camera, (2) parallax correction between the thermal and visible images, (3) aberration correction for the lenses, and (4) adjustment of the image view angles. An experimental setup demonstrates an efficient diagnosing using the merged images.