A new 28GHz 500kW gyrotron was developed for the electron cyclotron resonance heating system of tandem mirror GAMMA10. The maximum output power of 570 kW was obtained by experimental optimization. The efficiency of the gyrotron is 40~50% in the power range of 300~500 kW output with the optimization in spite of the restriction of the present magnet. The non-desirable influences caused by the leakage field of the magnet system of GAMM10 were minimized by the adjustment of gyrotron coi l parameters and the minor change of the gyrotron and magnetic shield design.