The invention provides a low S-containing steel sheet which has an S content reduced to about <=0.005% and can exhibit HAZ (heat affected zone) toughness not only when being subjected to ultrahigh heat input welding with a heat input volume, e.g., of >=60 kJ/mm, but also when being subjected to welding with a relatively high heat input volume, e.g., of >=15 kJ/mm. The steel sheet having a composition containing each of C, Mn, P, S, Al, Ti, N, Ca and O, and, if required, containing Si, and the balance iron with inevitable impurities, is characterized in that the content of solid solution N contained in the steel sheet is controlled to 0.0010 to 0.0060%.