We have investigated carrier injection characteristics of a pentacene field effect transistor (FET) by measuring the displacement currents of the device. Hole injection from Au source and drain electrodes was clearly observed at negative gate voltage. We found that the capacitance calculated from the observed displacement current is a good measure of the lateral spread of the injected carriers at organic/dielectric interfaces. This technique also gives the estimation of the total amount of the accumulated charges in the FET device by integrating the displacement current. The present results demonstrate that this method can successfully probe the behavior of the carriers in organic FETs(OFETs), which is crucial for elucidating the operating mechanism of the transistors. From the measurements of the C60 FET structure in the atmosphere, which shows no gate modulation, we revealed that this technique is also useful for failure analysis of OFETs.