An electric power steering device comprising: a hollow first shaft; a second shaft which is arranged coaxially with the first shaft; and a torsion bar which includes a pair of coupling shaft portions (35a, 35b) provided at both axial end portions thereof and a spring shaft portion provided between the pair of coupling shaft portions (35a, 35b), and which is arranged at an inner diameter side of the first shaft, wherein one of the coupling shaft portions is coupled to the first shaft and the other of the coupling shaft portions is coupled to the second shaft, respectively, such that torque can be transmitted, wherein at least one of the coupling shaft portions (35a, 35b) is press-fitted into a coupling hole formed in a counter-shaft which is one of the first and second shafts, to which the one of the coupling shaft portions is to be coupled, wherein a steering force from a steering wheel is applied to either one of the first and second shafts, wherein one of an outer peripheral surface of an axially intermediate portion of the spring shaft portion and an inner peripheral surface of an axially intermediate portion of the first shaft is provided with a diametrical extension portion (37a, 55) which is configured by any one of the spring shaft portion, the first shaft, and a bush provided on the spring shaft portion or the first shaft, and which extends in a diametrical direction more than portions adjacent to both axial sides thereof, wherein the diametrical extension portion (37a, 55) is in contact with or positioned closer to an opposite surface of the other of the outer peripheral surface of the axially intermediate portion of the spring shaft portion and the inner peripheral surface of the axially intermediate portion of the first shaft than the portions adjacent to both axial sides of the diametrical extension portion (37a, 55), wherein the diametrical extension portion (37a, 55) includes a large diameter portion configured by the axially intermediate portion of the spring shaft portion and having a larger outer diameter size than the portions adjacent to both axial sides thereof, and an outer peripheral surface of the large diameter portion is in contact with or positioned closer to an inner peripheral surface of the first shaft than outer peripheral surfaces of portions adjacent to both axial sides of the large diameter portion, and characterized in that the large diameter portion is provided at a plurality of axial positions of the spring shaft portion. a hollow first shaft; a second shaft which is arranged coaxially with the first shaft; and a torsion bar which includes a pair of coupling shaft portions (35a, 35b) provided at both axial end portions thereof and a spring shaft portion provided between the pair of coupling shaft portions (35a, 35b), and which is arranged at an inner diameter side of the first shaft, wherein one of the coupling shaft portions is coupled to the first shaft and the other of the coupling shaft portions is coupled to the second shaft, respectively, such that torque can be transmitted, wherein at least one of the coupling shaft portions (35a, 35b) is press-fitted into a coupling hole formed in a counter-shaft which is one of the first and second shafts, to which the one of the coupling shaft portions is to be coupled, wherein a steering force from a steering wheel is applied to either one of the first and second shafts, wherein one of an outer peripheral surface of an axially intermediate portion of the spring shaft portion and an inner peripheral surface of an axially intermediate portion of the first shaft is provided with a diametrical extension portion (37a, 55) which is configured by any one of the spring shaft portion, the first shaft, and a bush provided on the spring shaft portion or the first shaft, and which extends in a diametrical direction more than portions adjacent to both axial sides thereof, wherein the diametrical extension portion (37a, 55) is in contact with or positioned closer to an opposite surface of the other of the outer peripheral surface of the axially intermediate portion of the spring shaft portion and the inner peripheral surface of the axially intermediate portion of the first shaft than the portions adjacent to both axial sides of the diametrical extension portion (37a, 55), wherein the diametrical extension portion (37a, 55) includes a large diameter portion configured by the axially intermediate portion of the spring shaft portion and having a larger outer diameter size than the portions adjacent to both axial sides thereof, and an outer peripheral surface of the large diameter portion is in contact with or positioned closer to an inner peripheral surface of the first shaft than outer peripheral surfaces of portions adjacent to both axial sides of the large diameter portion, and characterized in that the large diameter portion is provided at a plurality of axial positions of the spring shaft portion.