A planar discharge vessel is composed of a front board 1 made of translucent glass plate and a rear board 2 made of translucent glass plate facing to the front board 1 with a certain distance, a side wall 3 and frit glass 4, which joins a periphery of the front board 1 and the rear board 2 via a side wall 3 to form a planar discharge vessel. Phosphor layers 5a, 5b are formed on an inner surface of the front board 1 and rear board 2 and convert an ultraviolet ray radiated from the discharge medium through discharge into visible light. The external electrode 6, to which a high voltage is applied and the external electrode 7, to which a low voltage is applied are arranged alternately on the outer surface of the rear board 2. A plurality of elongated spacers 8 are arranged at an equal pitch between the front board 1 and the rear board 2 keeping a distance between the front board 1 and the rear board 2 substantially constant and thereby preventing the lamp vessel from breaking due to a pressure difference between inside and outside of the discharge vessel. The linear spacer 8 is located within the maximum width of the external electrode 6, with which each electrode 6 is divided into two in the appearance. Each discharge space is lighted stably by the electrode 6, which is partly exposed to each of the discharge spaces divided by the linear spacer 8 and which is operated as the electrode 10 of each discharge space.