There are provided a surface light source having uniform and high brightness independently whether the light source is small or large while using semiconductor light emitting devices (LEDs), and an electrically illuminated signboard using the surface light source, which is thin and capable of uniform display and which operates with low electricity consumption. LEDs (12) of a dome type are arranged on four corners at a bottom surface of a box body (11) of a tray shape whose bottom surface is quadrilateral, whose upper side is open, on an inner surface of which a light reflection member (11b) is provided, and whose side walls (11c) are inclined outward. The LEDs (12) are arranged so as to mainly irradiate a region between a vertical plane on a diagonal line (N) of the bottom surface and one side wall (11c), and the region irradiated by each of the LEDs (12) of the dome type arranged on the four corners rotates individually and sequentially in a specific direction (same direction) in the box body (11). One or more of the surface light source units are arranged to construct an electrically illuminated signboard.