An ionic liquid which is high in ionic conductivity and high in safety without an anxiety of ignition or the like and an electrolyte composition containing the same are provided.The present invention concerns an electrolyte composition for photoelectric conversion device, containing a quaternary phosphonium salt ionic liquid represented by the following formula (1). A viscosity at 25° C. of this ionic liquid is preferably not more than 200 mPa·sec. In the formula (1), it is preferable that the alkoxyalkyl group is a methoxymethyl group, and all of the alkyl groups are an ethyl group. In the formula, R1 represents a linear alkyl group or a branched alkyl group each having from 1 to 6 carbon atoms; R2 represents a methyl group or an ethyl group; n represents an integer of from 1 to 6; and X represents N(SO2CF3)2 or N(CN)2.