The present invention provides a carboxyl-containing resin which is less susceptible to gelation and which can form a solder-resist layer having high electrical insulation properties. This carboxyl-containing resin has a structure derived from a biphenyl novolak epoxy resin by the addition of a carboxylic acid to at least some of the epoxy groups thereof, said carboxylic acid containing a polybasic acid as the essential component.