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In this study, electrical properties of ionic polymer-metal composite (IPMC) with an ionic liquid (IL) as a solvent, were evaluated in air. 1-ethyl-3-methyl-imidazolium tetrafluoroborate (EMIBF4) was used as an IL. As a first approximation for the initial several seconds after application of the step voltage (0.5 or 1.5 V), IPMC was explained by a simple equivalent circuit model expressed as a serial combination of the membrane resistor and the electrical double layer capacitor. The membrane resistance decreased with increasing environmental humidity. On the other hand, as for the double layer capacitance, at the lowest humidity of 20%RH, it was almost same between the two different voltages. In addition, the estimated membrane resistance agreed relatively well with the resistance of this equivalent circuit expressed as a parallel combination of a resistor and a capacitor which was derived from the complex impedance measurement in the frequencies higher than 5.0 Hz.