The present invention provides an electrolyte composition, comprising an electrolyte containing at least one kind of an imidazolium salt selected from the group consisting of 1-methyl-3-propyl imidazolium iodide, 1-methyl-3-isopropyl imidazolium iodide, 1-methyl-3-butyl imidazolium iodide, 1-methyl-3-isobutyl imidazolium iodide and 1-methyl-3-sec-butyl imidazolium iodide, a halogen-containing compound dissolved in the electrolyte, and a compound dissolved in the electrolyte and containing at least one element selected from the group consisting of N, P and S, the compound being capable of forming an onium salt together with the halogen-containing compound.