Photosynthetic reaction center (RC) proteins in photosynthetic bacteria are very interesting from the viewpoint of natural supramolecular electric devices. One of key factors for the enhancement in currents of RC protein devices is the achievement of low electron-injection barrier between proteins and metal electrodes. In this study, the electron transfer mediated by RC proteins between two gold electrodes modified with various self assemble monolayer (SAM) materials, was measured by use of conductive atomic force microscopy (CAFM). We found that, when both the Au(111) metal substrate and the gold-coated cantilever were modified with 2-mercaptopyridine, CAFM currents were especially enhanced.