To form a good latent image without shortening an exposure time even the time of high speed formation of an image and to also form a high grade image.

Noticing to the light emitting parts 11-02a, 11-02x at both end parts on a light emitting element chip 11-02, the light emitting part on the side of the second light emitting element chip array B opposed to the light emitting part 11-02a is the one 12-02k present in the vicinity of the central part of the light emitting element chip 12-02 and the light emitting part on the side of the light emitting element chip opposed to the light emitting part 1102x is the one 12-03j positioned in the vicinity of the central part of a light emitting element chip 12-03 in the same way. When the respective light emitting element chips have a light emitting profile on a projected side, since the low position of the light emitting profile of the second light emitting element chip array B comes to the high position of the light emitting profile of the first light emitting element array A on a latent image by this alternate arrangement, the quantity-of-light irregularity of the respective light emitting element chips can be reduced.