Steel having acid dew corrosion resistance, and exhaust gas flow path constituent member

The present invention provides a steel produced using an ordinary steel as a base material, is improved in sulfuric acid dew corrosion resistance without relying on the addition of Sb, and is preferably also improved in hydrochloric acid dew corrosion resistance. The steel having acid dew corrosion resistance according to the present invention comprises, in mass%, 0.005 to 0.200% of C, 0.20 to 0.80% of Si, 0.05 to 1.50% of Mn, 0.002 to 0.020% of P, 0.005 to 0.015% of S, 0.10 to 0.50% of Cu, 0.05 to 0.30% of Ni, 0.005 to 0.100% of Al, and 0 or more and less than 0.010% of Mo, with the remainder made up by Fe and impurities. Particularly when hydrochloric acid dew corrosion resistance is emphasized, the Mo content in the steel is adjusted to a value falling within the range from 0.005 to 0.030 mass%.