Electron-transfer proteins (ETPs) play important roles in biological functions such as photosynthesis, respiration, nitrogen fixation and others. In response to electron acceptance or donation of an ETP, atoms around the active center in it reorganize by an amount of energy λ. The reorganization energy λ is an important physical quantity determined by each ETP, regulating electron-transfer reactions by it. Let us measure currents through an ETP in scanning tunneling microscopy (STM) with and without photoirradiation. It is pointed out that a difference in the threshold bias voltage for rise of the current becomes a direct measure of λ. Simultaneously, the redox potential is obtained. Such measurements for various ETPs would provide a fundamental basis for understanding their functions in biological organisms.