Abstract: A steel sheet has a C content in the range of 0.04 to 0.25% by mass, a Si content in the range of 0.05 to 3% by mass, a Mn content in the range of 0.1 to 3% by mass, an Al content in the range of 0.01 to 2% by mass, a P content of 0.02% by mass or below and higher than 0% by mass, and a S content in the range of 0 to 0.005% by mass. The number of CaO.Al2O3 inclusions containing CaO, Al2O3, SiO2 and MgO, having a CaO content of 5% or above, a SiO2 content of 0.1% or above and an Al2O3 content of 60% or above, respectively, having major axes of 30 ?m or above, and contained in 1 kg of the steel sheet is seventy or below. The steel sheet is highly ductile and has high bore-expandability. A steel slab manufacturing method manufactures a steel slab for forming the steel sheet.