Electric double layer (EDL) capacitors are promising as next-generation energy accumulators if their capacitances and operation voltages are both high. However, only few electrolytes can simultaneously fulfill these two requisites. Here we report that an oligomeric ionic liquid such as IL4(TFSI) with four imidazolium ion units in its structure provides a wide electrochemical window of similar to 5.0 V, similar to monomeric ionic liquids. Furthermore, electrochemical impedance measurements using Au working electrodes demonstrated that IL4(TFSI) exhibits an exceptionally high EDL capacitance of similar to 66 mu F/cm(2), which is times as high as those of monomeric ionic liquids so far reported. We also found that an EDL-based field effect transistor (FET) using IL44T(FSI) as a gate dielectric material and SrTiO3 as a channel material displays a very sharp transfer curve with an enhanced carrier accumulation capability of similar to 64 mu F/cm(2), as determined by Hall-effect measurements.