To understand the physical meaning of threshold voltage in organic field-effect transistors (OFETs), we studied the threshold voltage (shift) dependence on gate-insulator thickness as well as active-layer thickness, by using pentacene OFETs with and without a dipole interlayer between pentacene active layer and SiO2 gate insulator. Results showed that the presence of dipole monolayer caused a large threshold voltage shift and there was a linear relationship between the threshold voltage shift and the layer thickness of pentacene as well as SiO2. Assuming the pentacene film is a dielectric layer and the threshold voltage in pentacene OFET is determined from a zero-electric-field condition at the gate insulator interface, we propose a model based on compensation of the local electric field in the vicinity of semiconductor and gate insulator interface. The model well accounts for both the large negative threshold voltage shift and the linear relation. These findings reveal the importance of interfacial electric field for analyzing organic devices.