The present invention provides: a steel for a welded structure to be used for a crude oil tank that exhibits excellent general and local corrosion resistance in crude oil corrosion caused in a steel oil tank and is capable of suppressing the formation of corrosion products (sludge) containing solid sulfur; a method for producing said steel; a crude oil tank; and a method for preventing a crude oil tank against corrosion. The present invention makes it possible to obtain general and local corrosion resistance in a crude oil tank environment and suppress the formation of corrosion products (sludge) containing solid sulfur by using a steel: containing, in mass, 0.001 to 0.2% C, 0.01 to 2.5% Si, 0.1 to 2% Mn, 0.03% or less P, 0.007% or less S, 0.01 to 1.5% Cu, 0.001 to 0.3% Al, 0.001 to 0.01% N as basic components and, further, 0.01 to 0.2% Mo and/or 0.01 to 0.5% W; and preferably satisfying the following expression; Solute Mo+Solute W≧0.005%.