In order to most effectively apply the noncontact atomic force microscopy (AFM) using frequency modulation (FM) technique as a science tool in a variety of fields such as surface science, it is very important to understand fully the imaging mechanisms of the noncontact AFM on various samples. The imaging mechanism has been investigated on a chemically reactive surface such as semiconductor surface and an insensitive surface such as pure metal surface and layered material surface. However, there is no report of a comparative study between a reactive surface and an insensitive surface using same tip. For instance, Si(100)2x1:H monohydride surface is that a Si(100)2x1 reconstructed surface is terminated by a hydrogen atom, and do not newly reconstruct as metal deposited semiconductor surface, and the surface structure hardly change. Thus, Si(100)2x1:H monohydride surface is one of most useful surface for a model system to investigate the imaging mechanism, experimentally and theoretically. However, there is no report for noncontact AFM imaging on Si(100)2x1:H monohydride surface, and whether the interaction between a very small atom as hydrogen and a tip apex is observable with noncontact AFM do not have been clarified. In the present experiments, we compared the noncontact AFM images obtained for the Si(100)2x1 reconstructed surface with that for Si(100)2x1:H monohydride surface to investigate the role of chemical reactivity on the surface. It is found that the distance between bright spots is increased by the hydrogen termination. On Si(100)2x1 reconstructed surface, the noncontact AFM atomically resolved the dangling bonds localized outside the silicon dimer with a clear contrast. On the other hand, on Si(100)2x1:H monohydride surface, the noncontact AFM atomically resolved the individual hydrogen atoms on top most layer.