Au/fullerene(C60)/SiO2/Si field effect transistor (FET) has been investigated to examine the behavior of carrier injection by using displacement current measurement (DCM) and infrared absorption spectroscopy in the multiple internal reflection (MIR-IRAS). At the first scan of DCM, an increasing displacement current due to electron injection was clearly observed above a threshold voltage in forward scan, while no flow-back of the injected electrons was observed, indicating that almost all electrons injected in the scan were trapped. The capacitance obtained from the increasing current suggests that the electrons injected from a Au electrode spread laterally along C60/SiO2 to some extent. At the further scans of DCM, the threshold voltage was shifted to higher voltage, and the lateral spread of the electron was quite suppressed due to the space charge of the filled traps. These results clearly demonstrate that DCM is a useful method to examine the behavior of carriers in field effect transistor (FET) devices. The spectral change in MIR-IRAS observed under bias voltage for electron injection is also reported.