To increase corrosion resistance to CO2 gas, particularly rusting resistance, to obviate the necessity of lining with inorganic material or austenitic stainless steel, and to improve economical efficiency by providing an alloy steel where specific amounts of Cr and Ni are combinedly added.

The steel has a composition consisting of, by mass ratio, ≤0.05% C, 0.050-0.50% Si, 0.3-1.5% Mn, ≤0.020% P, ≤0.010% S, 10.0-15.0% Cr, >2.0-6.0% Ni, 0.005-0.050% Al, and the balance Fe with inevitable impurities. If necessary, one or ≥2 kinds selected from 1.0-3.0% Cu, 0.050-1.0% Mo, 0.010-0.30% Nb, 0.010-0.20% V, 0.010-0.20% Ti, 0.0005-0.010% B, and 0.0005-0.010% REM are further incorporated into the above composition. Moreover, this steel is formed into steel plate by means of an ordinary manufacturing process and used.