To provide a polymer solid electrolyte which is used for whole solid type lithium secondary batteries and has high hydrolysis resistance and ion conductivity in a wide temperature range.

The polymer solid electrolyte comprises a new highly branched polymer in which oligoethylene oxide-containing repeating unit compounds are at least ether-bound. The polymer solid electrolyte has a high ion conductivity, a good mechanical strength, and good chemical stability in a wide temperature range.