The adsorption of aniline on a Si(111)7x7 surface was examined by scanning tunneling microscopy (STM). In the STM images at negative sample bias, aniline molecules were observed as bright ring-like features. We have found that aniline molecules adsorb on the center adatom site, with its amino group oriented to the rest atom site. The differences in reactivity of the various sites on the Si(111)7x7 surface to aniline were observed. The center adatom site is more reactive than the corner adatom site. With increasing aniline exposure, an aniline chain was observed. From the STM images obtained at various sample biases, this aniline chain is seen to have different electronic structure from an aniline molecule singly adsorbed on the Si(111)7x7 surface. This suggests that aniline molecules are associated with each other on the Si(111)7x7 surface.