  We show that the leptogenesis is automatic within the supersymmetric standard
model if there exist right-handed neutrinos with mass less than $H_{\it inf}$,
the expansion rate during the inflation. Its scalar component grows during the
inflation due to the quantum fluctuation, oscillates coherently after the
inflation, and its decay generates lepton asymmetry. The scenario is naturally
embedded into $SO(10)$ GUT without any modifications. We also discuss the case
with heavier right-handed neutrino.
