Abstract: An efficient brightness controller is disclosed, which can be applied to various discharge tubes different in length, diameters and/or the like. The controller includes a high frequency transformer 2 having a primary coil. First terminal (node N.sub.2) of the primary coil is connected to a power supply V.sub.DD1, and second terminal (node N.sub.1) is connected to an output element 3. The controller also includes an amplifying circuit 6 for amplifying the waveform of the voltage at the node N.sub.2. The controller further includes an ON-time/OFF-time control circuit 5 for setting the optimum ON time depending on the output voltage at the node N.sub.1 and the output from the amplifier 6, and then adjusting the OFF time on the basis of the set optimum ON time.