We have developed an optimal growth procedure for gas-source MBE production of a GaInP/GaAs heterointerface. The interface quality is crucial to obtaining high-performance GaAs solar cells with a GaInP barrier layer because minority carrier lifetime depends strongly on the interface structure. In situ Reflective High-Energy Electron Diffraction (RHEED) observation during the growth across the GaInP/GaAs heterointerface revealed that the phosphorus atoms are replaced by arsenic atoms in the near-interface region of the GaInP layer, and a transient layer acting as a carrier trap is formed. Introduction of a GaP layer into the interface was found to be effective in suppressing carrier loss. From Composition Analysis by Thickness Fringe-Transmission Electron Microscopy (CAT-TEM) images, it was also found that the optimum thickness of inserted GaP to avoid the generation of misfit dislocations is 1 nm.