To provide a steel sheet and plate of 0.5 to 4 mm in thickness which have breaking elongation of ≥30%, maintain the plastic workability of the steel sheet and plate and are enhanced in their strength.

The steel sheet and plate which have the breaking elongation of ≥30% and the thickness of 0.5 to 4 mm are heated by irradiating the entire or partial regions of the steel sheet and plate with the convergent laser beam and the molten and resolidified areas of 0.5 to 2 mm width penetrating the steel sheet and plate are formed in parallel at intervals of 10 to 50 mm, by which the plastic workability of the steel sheet and plate is maintained. In addition, their strength is enhanced, by which the improvement in the characteristics of the steel sheet and plate is made possible.