Provided are a cap seal with which molding of a seal material by hand can be omitted and variation in cap seal mounting quality can be minimized, a fastening structure provided with said cap seal, and a method for mounting a cap seal. This cap seal comprises: a plurality of cap parts (11) which have an opening (11b) in one end, inside of which a space (11a) is formed, and which have insulation; and a base part (12) which connects the peripheries at the ends of the cap parts (11) on the opening (11b)-sides together and which has insulation; the cap parts (11) accommodating seal members and protruding parts of fastening members inside the spaces (11a), and the opening (11b)-side surface of the base part (12) being fabricated into a mounting surface (16).