Conformation change of DNA under binding a DNA binding protein was studied using an AFM. An originally designed AFM mounted on an inverse microscope was used for imaging DNA. High mobility group (HMG) protein and pUC118 DNA were used as DNA binding protein and DNA for investigation in this study. The pUC118 DNA and its mixture with HMG2BJ were imaged by the AFM. In the AFM image of pUC118, DNA strands showed open structure with a few helix sites. The height of the helix part is 1.5 times higher than the ordinary DNA strand. Centrally AFM images of the mixture showed HMG-like particles on DNA strands. The height of the particle is 2-3 times higher than the DNA strands which showed the particles were not aggregating parts but HMG. The HMG bound on a crossing part of DNA which make a loop.