A method for lighting a flat fluorescent lamp for a large-sized backlight unit is disclosed, to prevent a discharge interference (scattering in fluorescent discharge) when lighting a plurality of groups of cylindrical electrodes being adjacent, in which an A.C. voltage is applied to one or two groups of cylindrical electrodes through introduction wires for lighting lamp in state of being not applied to adjacent one or two groups of cylindrical electrodes, so the plurality of groups of cylindrical electrodes are sequentially switched on and off in a time-division method at a speed not to generate the flicker of lamp.