A composition for forming a silver ion diffusion-suppressing layer includes an insulating resin and a compound including: a structure selected from the group consisting of a triazole structure, a thiadiazole structure and a benzimidazole structure; a mercapto group; and at least one hydrocarbon group optionally containing a heteroatom, with the total number of carbon atoms in the hydrocarbon group or groups being 5 or more. The composition for forming a silver ion diffusion-suppressing layer allows formation of a silver ion diffusion-suppressing layer capable of suppressing silver ion migration between metal interconnects containing silver or a silver alloy to improve the reliability on the insulation between the metal interconnects.