This paper reports the demonstration of the resonant inverter circuit for electrodeless fluorescent lamps using a high-voltage GaN-HEMT as a main switching device. A 620-V/1.4-A GaN-HEMT was designed and fabricated for power electronic applications. The dynamic on-resistance increased with current collapse phenomena was suppressed by the dual-FP structure and the switching operation could be realized under high applied voltage of over 350 V. As a high-voltage and high-frequency power supply application, a 13.56-MHz resonant inverter circuit for electrodeless fluorescent lamps was demonstrated using the fabricated device. The demonstrated circuit achieved high-voltage operation of 380 V, high-speed gate-switching of 4.5-7 ns, and lighting of the electrodeless lamp with an input power of 7-10 W. High-voltage operation realized a simple circuit composition for high-power efficiency and the discharge ignition of the lamp without the starting circuit. The power efficiency of the inverter circuit was over 90% with an input power of 9 W. These results show that high-voltage GaN devices are suitable for high-frequency switching applications under high-input voltages of several hundred volts.