A resonator of an electrodeless lighting system includes a body part formed in a prescribed shape; a transmission space part formed at one side of the body part and having an antenna of an microwave generator therein; a multi-step type resonating space part formed to be opened at one side, having a section gradually widening toward the opened side, receiving the microwave radiated from the antenna by means of an microwave feeder and resonating the microwave; and a stub formed at a certain height at an inner wall of the multi-step type resonating space part. Since the reflector for reflecting light has the enlarged size and is varied in its form to increase the amount of reflected parallel light, an illumination performance is enhanced. In addition, the impedance matching of the microwave exciting the gas filled in the electrodeless bulb and the resonance frequency are controllable.