A positive electrode-active material is provided for use in a positive electrode for a sodium secondary cell and a sodium secondary cell. Also provided is a positive electrode for a sodium secondary cell and a sodium secondary cell having a high energy density. The positive electrode-active material for a sodium secondary cell includes a composite metal oxide, which has an α-NaFeO2 type crystal structure and is denoted by Formula (1) described below: Naa(FewNixMny-zTiz)O2  (1), wherein a is greater than or equal to 0.6 and less than or equal to 1.0, w is greater than 0 and less than 0.5, x is greater than 0 and less than 0.5, y is greater than 0.03 and less than or equal to 0.5, z is greater than or equal to 0.03 and less than 0.5, and w+x+y=1.0, and y>z.