An ideal filament preheat technique in flourescence lamp is supplied by an adjusting circuit, an output circuit and a control circuit. At the filament preheating stage, an adequate preheating voltage is provided for the filament of the lamp tube, and the voltage cross over both ends of the lamp tube is reduced, thereby preventing glow current in preheating. When the preheating stage is finished and starting stage begins, the voltage between both ends of the lamp tube increases simultaneously, and then reaches a break-down voltage of the lamp tube, then the lamp tube is lit to enter a normal operation stage, the preheating voltage of the filament is cutoff under the control circuit, so as to save the filament power consumption. Therefore, the efficiency of the electronic ballast is improved, and the operation life of the flourescent lamp tube is extended.