To perform side printing for latent image which has no light quantity unevenness, nor pattern disturbances.

LED chips 22 are arranged on a substrate 21 in a direction orthogonally crossed with a film feeding direction. One LED chip 22 corresponds to one pixel for side printing of latent image. Each LED chip 22 is individually divided by a partition 23, above which a diffusion plate 24 is disposed. Light emitted by each LED chip 22 adjacent to each other is not mixed due to the partition 23, so that no light quantity unevenness nor pattern disturbances occur. Light emitted by each LED chip 22 is diffused within the respective pixel by means of the diffusion plate 24, making the light quantity within the pixel even. In this way, side printing of latent image having no light quantity unevenness, nor pattern disturbances is possible.