A lithium polymer secondary cell having a negative electrode, a positive electrode and, arranged between the electrodes, a polymer electrolyte layer, characterized in that the cell has been subjected to a heat treatment at a temperature of 40 to 60.degree. C. after the assembly thereof. The polymer cell is freed of or is reduced in the adverse effect of a residual monomer and initiator on the performance of the cell. The polymer electrolyte is formed by cross-linking polymerization of a precursor monomer for an ion conducting polymer in a non-aqueous electrolyte, and the residual monomer and initiator contained therein is consumed by the heat treatment.