A steel bar, wherein the steel bar consists of, as a chemical composition in terms of mass%: C: 0.30 to 0.80%; Si: 0.01 to 1.50%; Mn: 0.05 to 2.50%; Al: 0.010 to 0.30%; N: 0.0040 to 0.030%; P: 0.035% or less; S: 0.10% or less; Cr: 0 to 3.0%; Mo: 0 to 1.5%; Cu: 0 to 2.0%; Ni: 0 to 5.0%; B: 0 to 0.0035%; Ca: 0 to 0.0050%; Zr: 0 to 0.0050%; Mg: 0 to 0.0050%; Rem: 0 to 0.0150%; Ti: 0 to 0.150%; Nb: 0 to 0.150%; V: 0 to 1.0%; W: 0 to 1.0%; Sb: 0 to 0.0150%; Sn: 0 to 2.0%; Zn: 0 to 0.50%; Te: 0 to 0.20%; Bi: 0 to 0.50%; Pb: 0 to 0.50%, and a remainder including Fe and impurities, wherein a region which is along a line extending between a center of a cross section of the steel bar and a periphery of the cross section of the steel bar and which has a hardness higher than an average hardness in the line by Hv20 or more is a hardening region in the line, a minimum value of depth of the hardening regions in the 8 lines of which the angle is 45° is a minimum hardening depth in the cross section, and a maximum value of the depth of the hardening regions in the 8 lines is a maximum hardening depth in the cross section, wherein a difference between the maximum hardening depth in the cross section and the minimum hardening depth in the cross section is 1.5 mm or less, wherein a difference between a maximum value of the maximum hardening depth and a minimum value of the maximum hardening depth in the cross sections at 3 points which are separated from each other by 1650 mm parallel to a longitudinal direction of the steel bar is 1.5 mm or less, wherein a difference between a maximum value of the minimum hardening depth and a minimum value of the minimum hardening depth in the cross sections at the 3 points which are separated from each other by 1650 mm parallel to the longitudinal direction of the steel bar is 1.5 mm or less, wherein a structure in an area from a surface of the steel bar to a depth of 25% of a radius of the steel bar includes 10 area% or less of a ferrite and a remainder including one or more selected from a group consisting of a bainite and a martensite, wherein a boundary between grains which are adjacent to each other and of which an orientation difference is 15 degree or more is a grain boundary, and an equivalent circle diameter of an area surrounded by the grain boundary is a grain size, wherein an average value of the grain size of a bcc phase in the area from the surface of the steel bar to the depth of 25% of the radius of the steel bar is 1.0 to 10.0 μm, wherein an average value of the grain size of the bcc phase in an area from the depth of 50% of the radius of the steel bar to the center of the steel bar is 1.0 to 15.0 μm, wherein a hardness of a region of which a depth from the surface is 50 μm is Hv200 to Hv500, wherein a total decarburized layer thickness DM-T is 0.20 mm or less, and wherein the diameter of the steel bar is 19 to 120 mm. C: 0.30 to 0.80%; Si: 0.01 to 1.50%; Mn: 0.05 to 2.50%; Al: 0.010 to 0.30%; N: 0.0040 to 0.030%; P: 0.035% or less; S: 0.10% or less; Cr: 0 to 3.0%; Mo: 0 to 1.5%; Cu: 0 to 2.0%; Ni: 0 to 5.0%; B: 0 to 0.0035%; Ca: 0 to 0.0050%; Zr: 0 to 0.0050%; Mg: 0 to 0.0050%; Rem: 0 to 0.0150%; Ti: 0 to 0.150%; Nb: 0 to 0.150%; V: 0 to 1.0%; W: 0 to 1.0%; Sb: 0 to 0.0150%; Sn: 0 to 2.0%; Zn: 0 to 0.50%; Te: 0 to 0.20%; Bi: 0 to 0.50%; Pb: 0 to 0.50%, and a remainder including Fe and impurities, wherein a region which is along a line extending between a center of a cross section of the steel bar and a periphery of the cross section of the steel bar and which has a hardness higher than an average hardness in the line by Hv20 or more is a hardening region in the line, a minimum value of depth of the hardening regions in the 8 lines of which the angle is 45° is a minimum hardening depth in the cross section, and a maximum value of the depth of the hardening regions in the 8 lines is a maximum hardening depth in the cross section, wherein a difference between the maximum hardening depth in the cross section and the minimum hardening depth in the cross section is 1.5 mm or less, wherein a difference between a maximum value of the maximum hardening depth and a minimum value of the maximum hardening depth in the cross sections at 3 points which are separated from each other by 1650 mm parallel to a longitudinal direction of the steel bar is 1.5 mm or less, wherein a difference between a maximum value of the minimum hardening depth and a minimum value of the minimum hardening depth in the cross sections at the 3 points which are separated from each other by 1650 mm parallel to the longitudinal direction of the steel bar is 1.5 mm or less, wherein a structure in an area from a surface of the steel bar to a depth of 25% of a radius of the steel bar includes 10 area% or less of a ferrite and a remainder including one or more selected from a group consisting of a bainite and a martensite, wherein a boundary between grains which are adjacent to each other and of which an orientation difference is 15 degree or more is a grain boundary, and an equivalent circle diameter of an area surrounded by the grain boundary is a grain size, wherein an average value of the grain size of a bcc phase in the area from the surface of the steel bar to the depth of 25% of the radius of the steel bar is 1.0 to 10.0 μm, wherein an average value of the grain size of the bcc phase in an area from the depth of 50% of the radius of the steel bar to the center of the steel bar is 1.0 to 15.0 μm, wherein a hardness of a region of which a depth from the surface is 50 μm is Hv200 to Hv500, wherein a total decarburized layer thickness DM-T is 0.20 mm or less, and wherein the diameter of the steel bar is 19 to 120 mm.