A bonding agent having a great bond strength and enabling the improvement of the wettability of the bonding agent solution and the prevention of deterioration of cell performance. The bonding agent contains a thermoplastic resin, a solvent for dissolving the thermoplastic resin, and a neutral aprotic surfactant and is used as a bonding agent for bonding the active material layer joined to a current collector to the separator. This enables production of a thin arbitrarily shaped secondary cell having a high reliability and high charging and discharging efficiencies.