Abstract: The invention relates to a device for operating a fluorescent discharge lamp of the outer electrode type. As claimed in the invention a switching device is turned on and off by driver signals with a stipulated frequency, by which on the secondary side of a transformer the periodic voltage waveform shown at A in FIG. 1 is produced. This voltage waveform is applied to the fluorescent discharge lamp of the outer electrode type, and thus operation is effected. For the above described periodic voltage waveform, with respect to repetition period t 2 Wo<t, if the width at a voltage of the waveform of 0 V which has a single maximum peak value within one period is labeled WO, its full width at half maximum (the width between a and b in FIG. 1) being set to a stipulated value (for example, to a value of less than or equal to 2.5 .mu.sec). This makes it possible to maintain stable emission even if the pressure of the gas filled in the discharge lamp is increased.