Regardless of a relative installation position error between a light source and a photosensitive material, or of a deviation from a properly preset value of rotation speed of a photosensitive material, images are obtained with minimum loss of image quality and excellent reproducibility. The number m of light-emitting elements to make up a unit and the distance between adjacent pixels formed on the photosensitive surface are adjusted such that the width of the pixel groups is ⅓ mm or less, which is a period that the periodic image noise is not recognized as stripes.