A new sealing structure for helical heater terminal used in a combination microwave oven is suggested, and its sealing characteristics are investigated. The new structure is simply a conductor tube coaxially covering the heater at the input terminal. With an optimally-designed structure of this type, the transmission coefficient of the heater can be reduced to -41.6 dB at 2450 MHz. When installed in a test set which is running with an output of 500 W at 2450 MHz, the microwave power leaking out from the heater terminal is found to be less than 0.2 mW/cm2.