A device of detecting an abnormal state of a microwave oven having relays, a high voltage transformer, and a magnetron, including a voltage sensing part connected between the high voltage transformer and the magnetron to sense a voltage applied to the magnetron's both terminals; and a control part receiving the voltage between the magnetron's terminals from the voltage sensing part to determine an unloaded state where nothing is in a cooking chamber or an overheated state of the magnetron or the cooking chamber, and when determining that the oven is in the abnormal state, turning off the relays to stop the operation of the magnetron; and a method of detecting an abnormal state of a microwave oven including the steps of driving a magnetron to perform a user-selected cooking operation; detecting a voltage between the magnetron's both terminals; computing an average voltage by averaging the voltage between the magnetron's terminals; detecting an abnormal state by comparing the average voltage to a reference voltage; and if determining that the oven is in the abnormal state, stopping the operation of the magnetron.