A pressure-sensitive adhesive of the invention include a (meth)acryl-based polymer obtained by polymerization of a monomer component including 30 to 99.5% by weight of an alkyl (meth)acrylate having an alkyl group of 10 to 18 carbon atoms at an ester end and 0.5 to 50% by weight of a cyclic nitrogen-containing monomer. The pressure-sensitive adhesive has a satisfactory level of adhesive performance and can form a pressure-sensitive adhesive layer with a lower dielectric constant and reliable moisture resistance.