PURPOSE: To make uniform the quantity of light applied to a photo-sensitive body, over the whole width in the longitudinal direction of a head, by a simple structure.

CONSTITUTION: An insulating layer 8 for limiting the area of an exposed part of a transparent electrode 5 in the vicinity of a fluorescent pixel 5 is provided on the transparent electrode 4 and it is so constructed that the area of the exposed part of the transparent electrode 4 opposed to the central part in the longitudinal direction of filament cathodes 6 is wider than the area opposed to the opposite end parts thereof.