An as-deposited film with a Cr compositional gradient (22–15 at.% Cr) was immersed in 22.5% HNO3 for 15 hours. In the part of the film with initial Cr content in the range of 22–18 at.%, Cu dealloying resulted in sufficient Cu dealloying (final Cr content = 33–80 at.%) without film dissolution. Using the film with optimal initial composition Cu82Cr18, we successfully fabricated a nanoporous film with a pore size in the range of 20–40 nm. As a result of the formation of Cr2O3 during dealloying, this film was transparent and exhibited an insulation state. The novel nanoporous film is expected to be applied as a nanofilter in moisture-in-oil sensors.