To prevent generation of flaw on an external surface by specifying so that a radius of a corner part of slab has a specified ratio to a side length when a martensitic stainless steel slab is pierced by a press roll piercer.

A martensitic stainless steel represented with JIS standard SUS420J1 steel is pierced by a press roll piercer, a radius R of a corner part of slab is made to ≥14% to ≤20% of a side length H. Further, in case of working the corner part of rectangular slab to the radius R, when using a breakdown slab, the corner part is rolled to round by blooming, when using an as-cast slab, a radius is made to the corner part of a mold. By this method, when executing press roll piercing, generation of flaw on an external is greatly reduced.