Epitaxial growth mechanism of ZnSe under alternate supply of diethylzinc (DEZn) and dimethylselenium (DMSe) was studied by in situ optical monitoring of the growing surface. When H-2 was used as carrier gas, the time-dependent optical reflectance was found to exhibit a sawtooth pattern under DEZn supply. The sawtooth reflection signal was also observed in growth experiments using N-2 carrier gas when DEZn was provided after supply of neutral free radicals produced by laser photolysis of alkyl azide. Based on these experimental results, microscopic mechanism of growth enhancement by coexistent H-2 is discussed in terms of ab initio quantum chemistry.