Organic field-effect transistors (OFETs) were fabricated using pentacene thin layer, and the effects of inserted Lewis-acid thin layers on electrical properties were investigated. The OFETs have active layers of pentacene and vanadium pentoxide (V2O5) as a Lewis-acid layer. Typical source-drain current (I-DS) vs. source-drain voltage (V-DS) curves were observed under negative gate voltages (V(G)s) application, and the shift of the threshold voltage for FET driving (V-t) to positive side was also observed by V2O5 layer insertion, that is, -2.5 V for device with V2O5 layer and -5.7 V for device without V2O5 layer. It was thought that charge transfer (CT) complexes which were formed at the interface between pentacene and V2O5 layer were dissociated by the applied gate voltage, and the generated holes seem to contribute to drain current and the apparent V-t improvement.