A binder for electrode of lithium ion secondary battery, comprised of a copolymer composed of 15 to 80 weight % of units from ethylenically unsaturated monomer (A) whose homopolymerization yields a polymer soluble in N-methylpyrrolidone (NMP) and 20 to 85 weight % of units from ethylenically unsaturated monomer (B) whose homopolymerization yields a polymer insoluble in NMP, which copolymer exhibits a swelling degree of 4 or below, in an electrolyte obtained by dissolving LiPF.sub.6 in the concentration of 1 mol/liter into a solvent of 1:2 (volume ratio at 20.degree. C.) mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). This binder for electrode of lithium ion secondary battery enables obtaining an electrode having a flexible electrode layer excelling in binding properties with industrial advantage.