To provide a new electrolyte composition showing high ion conductivity and ion transference number and having remarkably low fluidity, and a nonaqueous electrolyte secondary battery of high capacity with excellent cycle stability.

This electrolyte composition contains fused salt of specific structure, silicon polymer and the salt of metal ion belonging to the group I or group II of the periodic table, and the nonaqueous electrolyte secondary battery contains the electrolyte composition. The fused salt is expressed by one of general formulas (1)-(3), wherein Q is an atomic group that can form the aromatic cation of 5-member ring or 6-member ring together with nitrogen atom, L11-L36 are each an alkylene group, an alkyleneoxy group, an alkenyleneoxy group or a bivalent connection group formed by plurally combining these groups; R11-R36 are each a hydrogen atom or a substituent which may be connected to each other to form a ring structure; n1 is 0 or 1 or more; X- is anion; and A is a nitrogen atom or phosphorus atom.