A steel wire for bolts, consisting of, in percentage by mass: C: 0.20 to 0.35%; Si: 0.01% to 2.0%; Mn: 0.3 to 1.5%; P: more than 0% and 0.020% or less; S: more than 0% and 0.020% or less; Cr: 0.10 to 1.5%; Al: 0.01 to 0.10%; B: 0.0005 to 0.005%; N: 0.001% to 0.015%; and at least one element of Ti: 0.02 to 0.10% and Nb: 0.02 to 0.10%, optionally at least one of the following elements (a) to (d): (a) one or more elements selected from the group consisting of: Cu: more than 0% and 0.3% or less, Ni: more than 0% and 0.5% or less, and Sn: more than 0% and 0.5% or less; (b) at least one element of Mo: more than 0% and 0.30% or less and V: more than 0% and 0.30% or less; (c) at least one element of Mg: more than 0% and 0.01% or less and Ca: more than 0% and 0.01% or less; and (d) at least one element of Zr: more than 0% and 0.3% or less and W: more than 0% and 0.3% or less, and the balance consisting of iron and inevitable impurities, wherein when a rate of a B content at D 0 /4 part in the steel wire for bolts is 100% where D 0 is a diameter of the steel wire for bolts, a ratio of a B content at a surface of the steel wire for bolts is 75% or less on average, and a difference between a maximum value and a minimum value of the ratio is 25% or less, and wherein a prior austenite crystal grain size number in a region from the surface to a depth of 100 μm of the steel wire for bolts is No. 8 or more. C: 0.20 to 0.35%; Si: 0.01% to 2.0%; Mn: 0.3 to 1.5%; P: more than 0% and 0.020% or less; S: more than 0% and 0.020% or less; Cr: 0.10 to 1.5%; Al: 0.01 to 0.10%; B: 0.0005 to 0.005%; N: 0.001% to 0.015%; and at least one element of Ti: 0.02 to 0.10% and Nb: 0.02 to 0.10%, optionally at least one of the following elements (a) to (d): (a) one or more elements selected from the group consisting of: Cu: more than 0% and 0.3% or less, Ni: more than 0% and 0.5% or less, and Sn: more than 0% and 0.5% or less; (b) at least one element of Mo: more than 0% and 0.30% or less and V: more than 0% and 0.30% or less; (c) at least one element of Mg: more than 0% and 0.01% or less and Ca: more than 0% and 0.01% or less; and (d) at least one element of Zr: more than 0% and 0.3% or less and W: more than 0% and 0.3% or less, (a) one or more elements selected from the group consisting of: Cu: more than 0% and 0.3% or less, Ni: more than 0% and 0.5% or less, and Sn: more than 0% and 0.5% or less; (b) at least one element of Mo: more than 0% and 0.30% or less and V: more than 0% and 0.30% or less; (c) at least one element of Mg: more than 0% and 0.01% or less and Ca: more than 0% and 0.01% or less; and (d) at least one element of Zr: more than 0% and 0.3% or less and W: more than 0% and 0.3% or less, and the balance consisting of iron and inevitable impurities, wherein when a rate of a B content at D 0 /4 part in the steel wire for bolts is 100% where D 0 is a diameter of the steel wire for bolts, a ratio of a B content at a surface of the steel wire for bolts is 75% or less on average, and a difference between a maximum value and a minimum value of the ratio is 25% or less, and wherein a prior austenite crystal grain size number in a region from the surface to a depth of 100 μm of the steel wire for bolts is No. 8 or more.