According to this invention, there is provided a binder containing a polyurethane resin having an urethane bond and formed by the reaction of a diisocyanate and a polyol, said resin containing on the average at least 2.3 isocyanate groups in a molecule, having a number average molecular weight of at least 2,000 but at most 10,000 and containing less than 20% of a component having a molecular weight of at most 1,000; a magnetic recording layer or back coat layer using same; and a magnetic recording medium having same. Said magnetic recording medium is excellent in magnetic characteristics, durability and running property.