An electrodeless discharge lamp apparatus includes an electrodeless discharge lamp, a microwave resonator, and a microwave coupler. The microwave resonator includes a conductive reflecting mirror having an opening, a conductive shield, and two opposing external electrodes provided substantially on a central axis of the reflecting mirror. The electrodeless discharge lamp is disposed between the opposing external electrodes. The focal point of the reflecting mirror is positioned between the opposing external electrodes. When microwave energy is supplied to the microwave resonator via the microwave coupler, a microwave resonant electric field occurs between the opposing external electrodes, whereby discharge of the electrodeless discharge lamp occurs.