A charging roller has a curved roller surface, and the shape of the roller surface is represented by Y/Y1=(X/X1)exp(α). A distance between a central portion of the roller body and a first arbitrary point on a rotation axis is denoted by X, a reduction in a radius at the first arbitrary point from the maximum radius at the central portion is denoted by Y, a distance between the central portion and a second arbitrary point on the rotation axis is denoted by X1, and a reduction in the radius at the second arbitrary point from the maximum radius at the central portion is denoted by Y1. The second arbitrary point is closer to an end portion of the roller body than the first arbitrary point.