A steering device comprising: an outer shaft that is disposed around a center axis and includes an inner space with a spline hole portion; and an inner shaft at least a part of which is disposed in the inner space, wherein the inner shaft includes a first spline shaft portion including a synthetic resin surface disposed around the center axis and includes a shaft portion that is disposed adjacent to one side of the first spline shaft portion in an axial direction parallel to the center axis, includes one end of the inner shaft disposed in the inner space, and includes a synthetic resin surface disposed around the center axis, wherein a distance between the center axis and the surface of the shaft portion is smaller than a distance between the center axis and the surface of the first spline shaft portion, wherein the spline hole portion and the first spline shaft portion are spline-fitted to each other, and the outer shaft and the inner shaft are relatively movable in the axial direction, wherein the shaft portion includes a second spline shaft portion with a spline having a tooth height smaller than a tooth height of a spline of the first spline shaft portion, wherein the inner space includes a first portion space disposed adjacent to one side of the spline hole portion in the axial direction, wherein a distance between the center axis and an inner surface of the first portion space is larger than a distance between the center axis and an inner surface of the spline hole portion, wherein the first spline shaft portion is disposed in the spline hole portion while the shaft portion is disposed in the first portion space, characterized in that the first portion space includes a tapered portion and a straight portion, the inner diameter of the tapered portion gradually increases toward one side of the axial direction from a boundary with the spline hole portion, the straight portion is disposed adjacent to one side of the tapered portion, and the inner diameter of the straight portion is uniform, and wherein the entire part of the first spline shaft portion is disposed in the spline hole portion while the second spline shaft portion is disposed in the tapered portion. an outer shaft that is disposed around a center axis and includes an inner space with a spline hole portion; and an inner shaft at least a part of which is disposed in the inner space, wherein the inner shaft includes a first spline shaft portion including a synthetic resin surface disposed around the center axis and includes a shaft portion that is disposed adjacent to one side of the first spline shaft portion in an axial direction parallel to the center axis, includes one end of the inner shaft disposed in the inner space, and includes a synthetic resin surface disposed around the center axis, wherein a distance between the center axis and the surface of the shaft portion is smaller than a distance between the center axis and the surface of the first spline shaft portion, wherein the spline hole portion and the first spline shaft portion are spline-fitted to each other, and the outer shaft and the inner shaft are relatively movable in the axial direction, wherein the shaft portion includes a second spline shaft portion with a spline having a tooth height smaller than a tooth height of a spline of the first spline shaft portion, wherein the inner space includes a first portion space disposed adjacent to one side of the spline hole portion in the axial direction, wherein a distance between the center axis and an inner surface of the first portion space is larger than a distance between the center axis and an inner surface of the spline hole portion, wherein the first spline shaft portion is disposed in the spline hole portion while the shaft portion is disposed in the first portion space, characterized in that the first portion space includes a tapered portion and a straight portion, the inner diameter of the tapered portion gradually increases toward one side of the axial direction from a boundary with the spline hole portion, the straight portion is disposed adjacent to one side of the tapered portion, and the inner diameter of the straight portion is uniform, and wherein the entire part of the first spline shaft portion is disposed in the spline hole portion while the second spline shaft portion is disposed in the tapered portion.