A surface light source device is provided to form uniformly luminance by adjusting the amount of discharge gas according to a change of temperature in a lighting state thereof. A surface light source device includes a light source body(110) and a discharge gas(115). The light source body includes an internal space for receiving the discharge gas and a barrier rib part for dividing the internal space into a plurality of discharge spaces. Each of the discharge spaces is filled with the discharge gas. The amount of discharge gas is proportional to the temperature of the light source body after driving the light source body. The discharge spaces are connected to each other in a line. The amount of discharge gas within each of the discharge spaces is increased in a direction opposite to a gravitational direction.
