To provide a polymer solid electrolyte with high electrolyte swelling and reduced electrolyte leakage even in a long time used.

The polymer solid electrolyte is a copolymer having network structure three-dimensionally cross-linked with a linear polymer formed with monofunctional monomers and multifunctional monomers cross-linking the linear polymer, and a polymerization composition having a large cross-linking point to point molecular weight (Mx) of 9,000 to 25,000 is used. The electrolyte in which the polymer contains an organic solvent and an electrolyte salt has high electrolyte swelling and is suitable for the electrolyte for a thin lithium ion battery using a laminated film as an outer package.