We have investigated a photosensor that consists of a field emission transistor (FET) utilizing the biocomponent of the photosystem I (PSI) protein complex for use in an imaging device. The PSI was immobilized on a gold electrode via the self-assembling monolayer (SAM) of 3-mercapto-1-propanesulfonic acid sodium salt to obtain a PSI-modified gold electrode. As for the PSI-modified gold electrode, the basic photoresponses originating from the excitation of PSI, including the photocurrent (106 nA) and the photoresponse of the open-circuit voltage (photo-Voc: 28.6 mV), were characterized. Then, the PSI-modified gold electrode was linked to the gate of the FET using a lead line, and the device was successfully driven by the photoelectric signals from the PSI like a voltage follower circuit. Further, we successfully demonstrated that the PSI-based FET acts as a photosensor in imaging devices.