An object of the present invention is to provide a boron-containing compound capable of forming an ion-conductive polyelectrolyte having high ion-conductive properties, and a polymer of said compound. According to the present invention, there are provided a polymerizable boron-containing compound of (formula 1), a polymer thereof, a polymer of a compound of (formula 2) and a compound of (formula 3), and a polyelectrolyte for electrochemical device comprising either of these polymers and at least one electrolyte salt: ##STR00001##