A stainless steel slab containing B, wherein a protecting material is joined onto at least two faces across the bloom from each other in a stainless steel bloom containing B of 0.3 2.5 mass %, being integrated into one-piece by forming a weld metal comprising a stainless steel with chemical composition that satisfies the relationship expressed by following formulas (1) (4), and a method to produce a steel product by rolling said slab. Further, it is preferable to interpose an insert material between above stainless steel bloom and above protecting material in bonding process. 15.ltoreq.Cr eq.ltoreq.30, (1) 4.ltoreq.Cr eq-Ni eq.ltoreq.17, (2) Cr eq=Cr+1.5 Si+Mo-5 B, (3) Ni eq=Ni+30 (C+N)+0.5 Mn (4)Herein, each symbol of a chemical element designates the content (mass %) of relevant chemical element contained in steel. Accordingly, it makes it possible to subject a stainless steel slab containing B to hot working without generating edge cracking, so as to be used as a source material of a neutron shielding container and a separator of a fuel cell.