A polymer for bonding the positive electrode and negative electrode of a lithium secondary battery, which includes a positive electrode, a negative electrode and an electrolyte solution, with a separator arranged between the positive electrode and the negative electrode. The polymer contains a cationically polymerizable monomer unit (A), a monomer unit (B) providing affinity to the electrolyte solution, a monomer unit (C) providing poor solubility to the electrolyte solution, and a monomer unit (D) containing an anionic or nonionic hydrophilic group. This polymer can be obtained through radical polymerization such as emulsion polymerization or suspension polymerization, and is characterized by having a dissolution rate into a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) [EC:DEC=5:5 (weight ratio)] of not more than 10% by weight.