An electric power steering device comprising: an input shaft having a fitting part near a front end thereof and applied with a steering force from a steering wheel; an output shaft rotatably supported inside a housing, coupled to the input shaft to be relatively rotatable within a predetermined angle range, having a torque detection encoder part provided at a part of an outer peripheral surface thereof, and applied with auxiliary power from an electric motor which is a generation source; a torsion bar provided at an inner diameter side of the input shaft and the output shaft with coaxially coupling the input shaft and the output shaft each other; and a torque detection sleeve which is arranged at an outer diameter side of the torque detection encoder part, and of which a rear end portion is externally fitted and fixed to the fitting part, wherein a worm wheel configuring a worm decelerator is externally fitted and fixed to a worm wheel fitting part of the output shaft, characterized in that a rolling bearing, which is configured to rotatably support the output shaft inside the housing, is externally fitted and fixed to a bearing fitting part, which is provided at a position axially adjacent to the worm wheel fitting part, of the output shaft, and wherein a side surface, which faces toward the worm wheel, of both axial side surfaces of an inner ring configuring the rolling bearing is contacted to a step part by which the bearing fitting part and the worm wheel fitting part continue to each other wherein a snap ring, which is configured to restrain the rolling bearing from being displaced in an opposite direction to the worm wheel with respect to an axial direction, is engaged to an engagement groove formed on the output shaft, and wherein a nitride layer is formed at a part, at which the torque detection encoder part is formed, of the outer peripheral surface of the output shaft, wherein the nitride layer is also formed on an outer peripheral surface of the worm wheel fitting part, on an outer peripheral surface of the step part, and at a part, at which the engagement groove is formed, of the outer peripheral surface of the output shaft. an input shaft having a fitting part near a front end thereof and applied with a steering force from a steering wheel; an output shaft rotatably supported inside a housing, coupled to the input shaft to be relatively rotatable within a predetermined angle range, having a torque detection encoder part provided at a part of an outer peripheral surface thereof, and applied with auxiliary power from an electric motor which is a generation source; a torsion bar provided at an inner diameter side of the input shaft and the output shaft with coaxially coupling the input shaft and the output shaft each other; and a torque detection sleeve which is arranged at an outer diameter side of the torque detection encoder part, and of which a rear end portion is externally fitted and fixed to the fitting part, wherein a worm wheel configuring a worm decelerator is externally fitted and fixed to a worm wheel fitting part of the output shaft, characterized in that a rolling bearing, which is configured to rotatably support the output shaft inside the housing, is externally fitted and fixed to a bearing fitting part, which is provided at a position axially adjacent to the worm wheel fitting part, of the output shaft, and wherein a side surface, which faces toward the worm wheel, of both axial side surfaces of an inner ring configuring the rolling bearing is contacted to a step part by which the bearing fitting part and the worm wheel fitting part continue to each other wherein a snap ring, which is configured to restrain the rolling bearing from being displaced in an opposite direction to the worm wheel with respect to an axial direction, is engaged to an engagement groove formed on the output shaft, and wherein a nitride layer is formed at a part, at which the torque detection encoder part is formed, of the outer peripheral surface of the output shaft, wherein the nitride layer is also formed on an outer peripheral surface of the worm wheel fitting part, on an outer peripheral surface of the step part, and at a part, at which the engagement groove is formed, of the outer peripheral surface of the output shaft.