To obtain a flat plate-shaped light source capable of maintaining stable light emission state for long time.

The flat plate-shaped light source is composed of a front panel 10 having a dielectric layer 14 covering a pair of discharge electrodes 12, 12 formed on one face, a back panel 20 having phosphor layer 22 on one face, and a barrier rib 30 arranged between the front panel 10 and the back panel 20 forming a discharging space 70 in which discharge gas is enclosed, wherein, the dielectric layer 14 on the front panel 10 and the phosphor layer 22 on the back panel are made to face each other. A stable glow discharge can be maintained and a phenomenon of discharge contraction is eliminated by forming a compact dielectric layer 14 on which leak current density between the discharge electrodes 12, 12 becomes not higher than 1×10-7A/cm2 at electric field strength of 1MV/cm.