PURPOSE: To remove unevenness in a light quantity distribution of a fluorescent tube without reducing a light quantity with a simple construction to obtain a uniform light quantity distribution to form an image with a little unevenness of an image density.

CONSTITUTION: A substrate 1 is securely held so that at least the top surface thereof is shaped into a substantially cylindrical surface which is gently projected on the side of a cathode. A distance between the substrate 1 and the cathode 5 is increased from the center part to the end part. Namely, where a distance at the center part is tc and a distance at the end part is te, a relation tc < te is held.