The present invention relates to a power supply device for a backlight and a liquid crystal device using the power supply device. The power supply device and the liquid crystal display device using the same according to the present invention can drive a fluorescent lamp while keeping luminance of the fluorescent lamp constant by applying voltages with different polarities to both ends of the fluorescent lamp, detecting a voltage proportional to a current flowing through the fluorescent lamp, and causing the detected voltage to be fedback. Therefore, there are advantages in that since a leakage current between the fluorescent lamp and a reflector is reduced, the luminance of the fluorescent lamp becomes uniform and the life of the fluorescent lamp is prolonged.