Disclosed are a polymer electrolyte having excellent chemical stability and high ion conductivity, and a battery using such a polymer electrolyte. Specifically disclosed is a polymer electrolyte (23) containing a polymer compound having a structure obtained by polymerizing polyvinyl acetal, and an electrolyte solution containing a solvent and an electrolyte salt. The solvent contains not less than 80% by mass of a carbonate ester obtained by mixing a cyclic compound such as ethylene carbonate and a chain compound such as methylethyl carbonate. The mass ratio between the cyclic compound and the chain compound in the carbonate ester is from 2:8 to 5:5. Consequently, there can be attained a high ion conductivity, and the solubility of polyvinyl acetal can be increased even when the ratio of the carbonate ester in the solvent is increased.