Ga clusters on a GaAs (0 0 1)(2 x 4)-As surface were successfully observed by a system in which scanning tunneling microscopy (STM) and molecular beam epitaxy (MBE) were equipped with not separated each chambers but one incorporate unit. It is found that the boundary between crystalline adatoms and magic clusters was found as a Ga trimer (N = 3) cluster. The dangling bonds of a Ga dimer (N = 2) are completely empty, satisfying electron counting heuristics, resulting no clear observation by the filled state imaging, and showing the crystalline adatoms. However, the Ga trimer is clearly observed. Moreover, the Ga trimer and heptamer (N = 7) have 9 and 21 electrons in molecular orbital states, which are closed to the magic numbers of spherical jellium, 8 and 20 respectively. It seems that this excess one electron becomes the support between the cluster and the substrate.