In situ UV-vis. absorption spectra of cytochrome c adsorbed on ITO electrode was observed with slab optical waveguide spectroscopy combining pulse potential step (PPS) between 0.3 and -0.45V vs. Ag/AgCl. The amount of cytochrome c adsorbed on ITO electrode was estimated from the amount of coulomb of the peaks in cyclic voltammogram to be about a monolayer coverage in this experimental condition. Spectral change between oxidized and reduced cytochrome c by PPS was finished in about 20msec with phosphate buffer solution. The results strongly proved that SOWG spectroscopy should be effective for in situ observation of ET reaction kinetics of surface adsorbed molecules.