Abstract: The electronic ballast includes a power source circuit for transforming a conventional AC power source into a stepped-up DC voltage and a half-bridge type inverted, in response to a pair of switching driving signals, for switching the stepped-up DC voltage to make a high-intensity discharge lamp operate. The ballast also includes a digital controller for generating the pair of the switching driving signals, which have a predetermined dead time respectively between a switching-ON time interval of the switching driving signals and a phase difference of 180 degrees therebetween, in a way that frequencies of the pair of the switching driving signals are gradually shifted into some other stepped levels in turn within a predetermined bandwidth by a predetermined time interval and such a frequency shifting operation is cyclically repeated, to be supplied to the half-bridge type inverter.