To economically produce the welded steel plate excellent in pitting corrosion resistance at a welded part without tempering.

A steel containing, by mass %, ≤0.20% C, ≤2.5% Mn, ≤0.015% S, 0.003-0.15%, Al and having a value of S/Mn of ≤0.01 is welded, successively, the welded part is subjected to a working of 1-5% at a working temp. of the A1 temp.-1000°C. In producing the steel, further, the steel can contain 0.1-1.0% Cu, 0.05-1.0% Ni or one kind or more of a prescribed content among Ti, Nb, Mo, B, Ca, REM.