A prehardened steel material comprising: 0.05≤C≤0.25 mass%, 0.01≤Si≤1.00 mass%, 1.20≤Mn≤1.80 mass%, 0.0002≤S≤0.3000 mass%, 0.30≤Cu≤1.80 mass%, 2.90≤Ni≤3.90 mass%, 0.08≤Cr≤3.20 mass%, 0.05≤Mo≤0.80 mass%, and 0.30≤Al≤1.50 mass%, and optionally, W≤1.00 mass%, Co≤2.00 mass%, B≤0.0050 mass%, Ca≤0.2000 mass%, Se≤0.50 mass%, Te≤0.100 mass%, Bi≤0.50 mass%, and Pb≤0.50 mass%, with a balance being Fe and unavoidable impurities, the upper limits for the unavoidable impurities being set as: P≤0.05 mass%, N≤0.015 mass%, O≤0.01 mass%, V≤0.12 mass%, Nb≤0.05 mass%, Ta≤0.05 mass%, Ti≤0.12 mass%, Zr≤0.12 mass%, Mg≤0.02 mass%, or REM≤0.10 mass%; wherein the prehardened steel material has: a cross-sectional size of 350 mm or more in width and 350 mm or more in height, wherein the width and the height refer to dimensions of the cross-section perpendicular to a direction in which the length of the material finally extends when the material undergoes hot plastic working, wherein a smaller value is defined as the height, and wherein the length direction is the fiber direction; a hardness of 34 to 43 HRC, an average value of prior austenite grain size over the entire cross-section as measured utilizing the method described in the description being 85 μm or less, and an average value of impact value over the entire cross-section being 18 J/cm 2 or higher, wherein the impact value is obtained utilizing the method described in the description, and wherein the average value of 10 test pieces is defined as average value of impact value. 0.05≤C≤0.25 mass%, 0.01≤Si≤1.00 mass%, 1.20≤Mn≤1.80 mass%, 0.0002≤S≤0.3000 mass%, 0.30≤Cu≤1.80 mass%, 2.90≤Ni≤3.90 mass%, 0.08≤Cr≤3.20 mass%, 0.05≤Mo≤0.80 mass%, and 0.30≤Al≤1.50 mass%, and optionally, W≤1.00 mass%, Co≤2.00 mass%, B≤0.0050 mass%, Ca≤0.2000 mass%, Se≤0.50 mass%, Te≤0.100 mass%, Bi≤0.50 mass%, and Pb≤0.50 mass%, with a balance being Fe and unavoidable impurities, the upper limits for the unavoidable impurities being set as: P≤0.05 mass%, N≤0.015 mass%, O≤0.01 mass%, V≤0.12 mass%, Nb≤0.05 mass%, Ta≤0.05 mass%, Ti≤0.12 mass%, Zr≤0.12 mass%, Mg≤0.02 mass%, or REM≤0.10 mass%; wherein the prehardened steel material has: a cross-sectional size of 350 mm or more in width and 350 mm or more in height, wherein the width and the height refer to dimensions of the cross-section perpendicular to a direction in which the length of the material finally extends when the material undergoes hot plastic working, wherein a smaller value is defined as the height, and wherein the length direction is the fiber direction; a hardness of 34 to 43 HRC, an average value of prior austenite grain size over the entire cross-section as measured utilizing the method described in the description being 85 μm or less, and an average value of impact value over the entire cross-section being 18 J/cm 2 or higher, wherein the impact value is obtained utilizing the method described in the description, and wherein the average value of 10 test pieces is defined as average value of impact value. P≤0.05 mass%, N≤0.015 mass%, O≤0.01 mass%, V≤0.12 mass%, Nb≤0.05 mass%, Ta≤0.05 mass%, Ti≤0.12 mass%, Zr≤0.12 mass%, Mg≤0.02 mass%, or REM≤0.10 mass%; wherein the prehardened steel material has: a cross-sectional size of 350 mm or more in width and 350 mm or more in height, wherein the width and the height refer to dimensions of the cross-section perpendicular to a direction in which the length of the material finally extends when the material undergoes hot plastic working, wherein a smaller value is defined as the height, and wherein the length direction is the fiber direction; a hardness of 34 to 43 HRC, an average value of prior austenite grain size over the entire cross-section as measured utilizing the method described in the description being 85 μm or less, and an average value of impact value over the entire cross-section being 18 J/cm 2 or higher, wherein the impact value is obtained utilizing the method described in the description, and wherein the average value of 10 test pieces is defined as average value of impact value. a cross-sectional size of 350 mm or more in width and 350 mm or more in height, wherein the width and the height refer to dimensions of the cross-section perpendicular to a direction in which the length of the material finally extends when the material undergoes hot plastic working, wherein a smaller value is defined as the height, and wherein the length direction is the fiber direction; a hardness of 34 to 43 HRC, an average value of prior austenite grain size over the entire cross-section as measured utilizing the method described in the description being 85 μm or less, and an average value of impact value over the entire cross-section being 18 J/cm 2 or higher, wherein the impact value is obtained utilizing the method described in the description, and wherein the average value of 10 test pieces is defined as average value of impact value.