Inter-surface diffusion of Ga on a non-planar surface in GaAs molecular beam epitaxy (MBE) was studied by using microprobe-reflection high-energy electron diffraction/scanning electron microscopy MBE (mu-RHEED/SEM MBE). It was demonstrated that two-face or three-face inter-surface diffusion occurs depending on the length of the side surface of a mesa. By employing a long side surface, it was possible to realize pure two-face inter-surface diffusion and it was found that an inversion of its direction occurs twice as the arsenic pressure is increased. Fabrication of the pyramidal structure by making use of inter-surface diffusion was performed. The growth process was monitored in situ by mu-RHEED/SEM MBE. It was found that the decrease of the top size of the grown pyramidal structure occurs more quickly as the growth temperature is increased and as the arsenic pressure is decreased. This effect has been explained in terms of the enhancement of inter-surface diffusion under the chosen growth conditions.