We fabricated a photo field effect transistor (photo FET) having a poly (N-vinyl carbazole) (PVK) insulator layer. PVK was employed as a photosensitive insulator material for the Photo FET. In this photo FET, illumination of a blue light to the PVK insulator drastically improved the field effect mobility and the gate-switching ratio. This result would be due to that the photo-generated carriers in the PVK layer were effectively accumulated in the gate capacitor of the photo FET.