In ultraviolet-photoelectron spectroscopy on a Si(100) surface during solid-source or gas-source molecular-beam epitaxy (MBE), intensity oscillations of photoelectrons from the valence-band surface states are observed. Recent studies by authors [Enta et al., Surf. Sci. 313, L797 (1994)] have revealed that the photoelectron intensity oscillations (PIOs) most probably originate from an alternation between the 2x1 and the 1x2 surface reconstructions during growth. This model consequently suggests that the period of the oscillations corresponds to the growth time of 2 ML. To confirm this relation, reflection high-energy electron diffraction (RHEED) intensity oscillation was measured during gas-source MBE, using Si2H6 under identical growth conditions with those used for PIO observations. The oscillation period at the half-order diffraction spots of RHEED agreed well with that of PIO at various Si2H6 pressures, providing a direct support for the above interpretation for the origin of PIO.