beta-DiCC[Ni(dmit)(2)] (dmit: 1,3-dithiole-2-thione-4,5-dithiolato and DiCC: 3,3'-dihexyloxacarbocyanine) is a charge-transfer salt with redox activity. When voltages are applied, it brings about a reversible, hysteretic, solid-state-electrolytic reaction exhibiting a capacitor-like electric response. Based on the electron spin resonance spectra, the resistivity behavior, and the tight-binding band calculation, it is indicated that one can finely and reversibly control the redox equilibrium between DiCC(+) and [Ni(dmit)(2)](-) molecules by applying voltages.