The present invention relates to a device for preventing microwave leakage for a microwave oven. The present invention comprises a cavity assembly formed with an opening portion through which heating energy generated in an energy generating means is supplied into a cooking chamber provided within the cavity assembly; a cover provided on a side of the cavity assembly to cover the opening portion; and a choke member provided between the cavity assembly and the cover to prevent microwave supplied into the cooking chamber from leaking through a gap between the cavity assembly and the cover. According to the present invention, there is an advantage in that microwave is effectively prevented from leaking through a gap, which is formed when installing a heating means for providing a heating source in addition to the microwave.