To provide an easy method for improving simultaneously both tensile strength and fracture toughness of high-strength ferrite/martensitic steel in a low temperature region.

A method includes: a first step of enhancing tensile strength of a steel material by subjecting ferrite/martensitic steel as a raw material to cold rolling until the ferrite/martensitic steel decreases in thickness by 20% or more; and then, a second step of introducing artificial fatigue precracks in the ferrite/martensitic steel, having been subjected to the cold rolling, to form many secondary microcracks in the peripheral areas of the precracks. The area, where the secondary microcracks are formed, is cut out of the ferrite/martensitic steel, which is thus obtained, and used, thereby simultaneously and greatly enhancing the tensile strength and fracture toughness of such steel material as is always exposed to, for example, about -165°C.