To provide a solid-solution positive electrode active material comprising a complex oxide, which is capable of maintaining high reversible capacity and reducing irreversible capacity, high in capacity and excellent in initial charge/discharge efficiency, and capable of being suitably used for an electrochemical device typified by a lithium ion secondary battery.

A solid-solution complex oxide is used as a positive electrode active material, the solid-solution complex oxide being represented by a composition formula: [Li_1.5][Li_{{0.5{1-x}-ny} (n-1)y}M'_nyMn_1-xM_1.5x]O₃ (in the formula, x satisfies an expression of 0.1≤x≤0.5, M is represented as NiαCoβMnγ, where 0<α≤0.5, 0≤β≤0.33 and 0<γ≤0.5 are satisfied, and M' represents at least on element selected from the group consisting of Mg, Zn, Al, Fe, Ti and V while the valence n thereof is 2 or more and 5 or less, and 0<ny<0.5 is satisfied).