A one-dimensional image sensor has N number of photosensitive element arrays arranged parallel to the fast scanning direction, N being a positive integer greater than 1. The adjacent N number photosensitive element arrays are separated from one another by a distance equal to the width of D number arrays in the slow scanning direction. If the moving speed of the one-dimensional image sensor in the slow scanning direction is set to M times as high as an ordinary speed, the distance between the adjacent N number photosensitive element arrays expressed in terms of the width of the D number arrays satisfies the following equation: D=Ma/n where a.noteq.nN, n being a positive integer equal to or smaller than N.