A composition for reducing a copper oxide layer to metallic copper so as to facilitate bonding a resin to the metallic copper is disclosed. The composition is an aqueous reducing solution containing a cyclic borane compound. Examples of such cyclic borane compounds include those having nitrogen or sulfur as a ring-forming member, such as morpholine borane, piperidine borane, pyridine borane, piperazine borane, 2,6-lutidine borane, 4-methylmorpholine borane, and 1,4-oxathiane borane, and also N,N-diethylaniline borane.