Cytochrome c is one of the most extensively studied proteins because of their unique functionalities including electron transfer (ET). ET reaction of cytochrome c is slow, thus direct electron transfer (DET) reaction between proteins and electrodes is rarely observed. We have invented slab optical waveguide (SOWG) spectroscopy by which in situ observation of UV-vis. absorption spectra from adsorbed molecules on solid/liquid interfaces under monolayer coverage can be performed. As a great advantage, SOWG spectral change can bring us molecular functionality. In this presentation, we will report, we will report DET reaction of cytochrome c adsorbed on ITO electrode by SOWG spectral change due to electrode potential scan, and that cytochrome c adsorbed on ITO electrodes kept ET activity. without addition of any promoters or mediators, and surface modification.