A flat fluorescent lamp is disclosed, wherein rows of U-shaped grooves are formed in a front glass substrate, rows of wavy parallel grooves are formed in a rear glass substrate, and the U-shaped grooves and the wavy parallel groves are made to be perpendicular to each other. Also, fluorescent substance coating films are interposed so as to be in contact with each other on the inner surfaces, and this realizes a structure resistant to an external air pressure. Furthermore, fluorescent discharge is produced inside a tunnel-shaped cavity between the rows of U-shaped grooves, and reduction in luminance at the lower side of the U-shaped grooves is compensated by illumination of the wall of the U-shaped grooves, where the reduction in brightness is due to non-illuminating sections being caused because the U-shaped grooves and the wavy parallel grooves are in contact with each other. Thus, a flat fluorescent lamp with uniform surface luminance is realized. Further, a heater wire is formed on the lower side in the lamp to heat and vaporize mercury received on the lower side in the lamp, realizing a flat fluorescent lamp capable of illuminating brightly even at a low temperature.