A method for producing a steel part, which comprises the steps of: preparing a built-up steel sheet comprising a steel sheet comprising: C: 0.15 to 0.5% by mass, Si: 0.10 to 3% by mass, Mn: 0.5 to 5% by mass, P: 0.05% by mass or less (excluding 0%), S: 0.05% by mass or less (excluding 0%), Al: 0.01 to 1% by mass, B: 0.0002 to 0.01% by mass, Ti: 0.005 to (3.4 + 0.1) % by mass (in which represents a content of N (% by mass)), and N: 0.001 to 0.01% by mass, and optionally any one or more of: (a) 0.1% by mass or less (excluding 0%) in total of one or more of V, Nb and Zr, (b) 0.01 to 2% by mass in total of Cr and/or Mo, (c) 0.01 to 0.5% by mass in total of Ni and/or Cu, and (d) 0.01 % by mass or less (excluding 0%) in total of one or more of Mg, Ca and REM, with the balance being iron and inevitable impurities, and one or more built-up portions provided on the steel sheet; hot-forming the built-up steel sheet at a temperature of an Ac3 point or higher of the steel sheet; and cooling the hot-formed built-up steel sheet to a temperature of an Ms point or lower of the steel sheet such that an area ratio of martensite in a metal structure of the steel sheet is 70% or more, wherein the step of preparing a built-up steel sheet comprises: preparing the steel sheet, and welding a welding wire on the steel sheet to form the built-up portions, and wherein the welding wire comprises: C: 0.10 to 1.00% by mass, Si: 0.2 to 1.20% by mass, Mn: 1.0 to 20.0% by mass, Cr: 1.0 to 30.0% by mass, and Mo: 0.30 to 1.00% by mass, and optionally any one or more of: Ni: 3.00% by mass or less (excluding 0%), Ti: 0.20% by mass or less (excluding 0%), Cu: 0.50% by mass or less (excluding 0%), S: 0.020% by mass or less (excluding 0%), Co: 1.00% by mass or less (excluding 0%), V: 1.00% by mass or less (excluding 0%), W: 2.00% by mass or less (excluding 0%), and B: 0.020% by mass or less (excluding 0%), with the balance being iron and inevitable impurities. preparing a built-up steel sheet comprising a steel sheet comprising: C: 0.15 to 0.5% by mass, Si: 0.10 to 3% by mass, Mn: 0.5 to 5% by mass, P: 0.05% by mass or less (excluding 0%), S: 0.05% by mass or less (excluding 0%), Al: 0.01 to 1% by mass, B: 0.0002 to 0.01% by mass, Ti: 0.005 to (3.4 + 0.1) % by mass (in which represents a content of N (% by mass)), and N: 0.001 to 0.01% by mass, and optionally C: 0.15 to 0.5% by mass, Si: 0.10 to 3% by mass, Mn: 0.5 to 5% by mass, P: 0.05% by mass or less (excluding 0%), S: 0.05% by mass or less (excluding 0%), Al: 0.01 to 1% by mass, B: 0.0002 to 0.01% by mass, Ti: 0.005 to (3.4 + 0.1) % by mass (in which represents a content of N (% by mass)), and N: 0.001 to 0.01% by mass, and optionally any one or more of: (a) 0.1% by mass or less (excluding 0%) in total of one or more of V, Nb and Zr, (b) 0.01 to 2% by mass in total of Cr and/or Mo, (c) 0.01 to 0.5% by mass in total of Ni and/or Cu, and (d) 0.01 % by mass or less (excluding 0%) in total of one or more of Mg, Ca and REM, (a) 0.1% by mass or less (excluding 0%) in total of one or more of V, Nb and Zr, (b) 0.01 to 2% by mass in total of Cr and/or Mo, (c) 0.01 to 0.5% by mass in total of Ni and/or Cu, and (d) 0.01 % by mass or less (excluding 0%) in total of one or more of Mg, Ca and REM, with the balance being iron and inevitable impurities, and one or more built-up portions provided on the steel sheet; hot-forming the built-up steel sheet at a temperature of an Ac3 point or higher of the steel sheet; and cooling the hot-formed built-up steel sheet to a temperature of an Ms point or lower of the steel sheet such that an area ratio of martensite in a metal structure of the steel sheet is 70% or more, wherein the step of preparing a built-up steel sheet comprises: preparing the steel sheet, and welding a welding wire on the steel sheet to form the built-up portions, and preparing the steel sheet, and welding a welding wire on the steel sheet to form the built-up portions, and wherein the welding wire comprises: C: 0.10 to 1.00% by mass, Si: 0.2 to 1.20% by mass, Mn: 1.0 to 20.0% by mass, Cr: 1.0 to 30.0% by mass, and Mo: 0.30 to 1.00% by mass, and optionally C: 0.10 to 1.00% by mass, Si: 0.2 to 1.20% by mass, Mn: 1.0 to 20.0% by mass, Cr: 1.0 to 30.0% by mass, and Mo: 0.30 to 1.00% by mass, and optionally any one or more of: Ni: 3.00% by mass or less (excluding 0%), Ti: 0.20% by mass or less (excluding 0%), Cu: 0.50% by mass or less (excluding 0%), S: 0.020% by mass or less (excluding 0%), Co: 1.00% by mass or less (excluding 0%), V: 1.00% by mass or less (excluding 0%), W: 2.00% by mass or less (excluding 0%), and B: 0.020% by mass or less (excluding 0%), Ni: 3.00% by mass or less (excluding 0%), Ti: 0.20% by mass or less (excluding 0%), Cu: 0.50% by mass or less (excluding 0%), S: 0.020% by mass or less (excluding 0%), Co: 1.00% by mass or less (excluding 0%), V: 1.00% by mass or less (excluding 0%), W: 2.00% by mass or less (excluding 0%), and B: 0.020% by mass or less (excluding 0%),