To provide a fastening tool that sufficiently suppress deformation breakage due to torque during fastening and contains steel having sufficient surface hardness.SOLUTION: The fastening tool is characterized in that the composition of the components is C: 0.55 to 0.70 mass%, Si: 1.50 to 2.50 mass%, Mn: 0.50 to 1.50 mass%, P: over 0 mass% to 0.050 mass% or smaller, S: over 0 mass% and 0.050 mass% or smaller, Ni: 0.10 to 0.60 mass%, Cr:0.50 to 1.50 mass%, V: 0.02 to 0.20 mass%, Al: over 0 mass% to 0.020 mass% or smaller, N: over 0 mass% to 0.010 mass% or smaller, and the balance: Fe and inevitable impurity, and the metallurgical structure consists of tempered martensite, and contains steel satisfying the following formula (1). Ci-Cs≤0.130(mass%) (1), Ci is an average value of the C concentration (mass%) at a position of 1/3 to 2/3 of a length to the center of gravity of the cross-section from an arbitrary point of the steel surface in an arbitrary cross-section vertical to a longitudinal direction of the steel, and Cs is the C concentration (mass%) at a position of 0.1 mm toward the center of gravity of the cross-section from the arbitrary point.SELECTED DRAWING: None