The sensory transduction processes of blue light in guard cells have been suggested the involvement of Ca$^{2+}$/calmodulin-dependent myosin light chain kinase (MLCK) or MLCK-like proteins. The source of Ca$^{2+}$ required for the signal transduction process was investigated in guard cell protoplasts (GCPs). The GCPs showed the typical H$^+$ pumping activity by blue light (200 $mu$mol m$^{-2}$ s$^{-1}$) and fusicoccin (10 $mu$M) under background red light (600 $mu$mol m$^{-2}$ s$^{-1}$). The blue light-dependent H$^+$ pumping was not significantly affected by the externally changed Ca$^{2+}$ concentrations. The addition of 1 mM Ca$^{2+}$ in the bathing medium ratherly inhibited the H$^+$ pumping. In contrast, the blue light-dependent H$^+$ pumping was inhibited by caffeine and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (BHQ), inhibitor of C$^{2+}$-ATPase in endoplasmic reticulum (ER) without inhibiting the H $^+$ pump. The inhibition by caffeine and BHQ was fully reversible. The extent of inhibition by caffeine and BHQ was larger when they were added together than when added separately. The results suggest that Ca$^{2+}$ required for the blue light-dependent H$^+$ pumping may be released from the intracellular Ca$^{2+}$ stores, probably ER in guard cells.