PURPOSE: To produce an outer shell member of a pocket watch, etc., having sufficient strength, by preparing a ferritic single-phase steel which contains γ-phase-forming elements, such as Ni, and α-phase-forming elements, such as Cr, and in which Ni equivalent, Cr equivalent, and Cr content are respectively specified.

CONSTITUTION: A ferritic single-phase steel, having a nominal composition which contains γ-phase-forming elements selected from among Ni, C, N, and Mn and α-phase-forming elements selected from among Cr, Mo, Si, and Nb and in which nickel equivalent Ni*, defined by equation Ni*=Ni%+30(C+ N)%+0.5Mn%, chromium equivalent Cr*, defined by equation Cr*=Cr%+Mn%+1.5Si+0.5Nb, and Cr content are regulated to ≤2%, 17.5-22%, and ≤18.5%, respectively, is prepared. At this time, it is preferable that the ferritic single- phase has a composition consisting of 0.07% of (C+N), <0.8% Si, 17-18.5% Cr, 1-4% Mo, <1% of (Nb+Ti), <1.2% Mn, <0.5% S, <1% Ni, and the balance iron with inevitable impurities. By this method, the outer shell member, having sufficient strength and free from allergic problem, can be obtained.