NOVELTY - A polymer gel electrolyte, comprises an aprotic organic solvent, a carrier salt and a sulfur-containing organic compound (-O-SO2-).    USE - As a polymer gel electrolyte useful in polymer secondary battery (claimed).    ADVANTAGE - The polymer gel electrolyte containing a sulfur-containing organic compound such as a sulfonic acid ester, can hold back the generation of gases due to charge/discharge during initial charging, thereby making sure improved rate and cycle performances. The polymer secondary battery has an improved capacity sustenance rate in the cycle performance and a good enough effect on prevention of cell swelling, and can hold back a resistance increase during storage. The secondary battery covered around by a flexible film comprising a metal foil and a synthetic resin film, it is possible to hold back resistance increases and prevent the battery from swelling by reason of the generation of gases. Thus, the polymer gel electrolyte is effectively applied to not only small-size polymer secondary battery of small size for portable equipment but also large-size ones for automobile applications. The polymer gel electrolyte is also effectively applied to a lithium polymer battery using as a negative electrode material scaly graphite so far taken to be unsuitable for a negative electrode material, because the generation of gases upon charge/discharge can be held back. The polymer battery using the polymer gel electrolyte has good rate performance, has high percentage capacity sustenance with little or no swelling of the covering film, even after subjected to repeated charge/discharge cycles, and is minimized in terms of an increase in resistivity after storage. The polymer gel electrolyte can be applied to not only batteries for small-size portable equipment but also large-size batteries for automobiles or the like.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a polymer secondary battery, comprising the polymer gel electrolyte, and further comprising a positive electrode including a lithium-containing composite oxide as a positive electrode active substance and a negative electrode containing as a negative electrode active substance a substance capable of inserting or deinserting lithium.