The present invention provides a binder for a lithium ion secondary battery electrode that has good adhesive properties and a low degree of swelling with respect to an electrolytic solution. There is used a binder for a lithium ion secondary battery electrode, which comprises a polyvinyl acetal-based resin (A) containing structural units represented by chemical formulae (1) to (3): (R1 is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms), and (R2 is a functional group containing an ethylenically unsaturated bond).