Disclosed herein is a microwave oven, in which a tray within a cooking room is rectilinearly reciprocated in a lengthwise direction of the cooking room and is rotated, simultaneously, to cook food. This is achieved using a gear assembly including a main gear and a planetary gear. The gear assembly includes the main gear and the planetary gear, which are engaged with each other and are rotated. A diameter of the main gear and a diameter of the planetary gear may be equal to each other, and the main gear and the planetary gear respectively include eccentric shafts. The gear assembly effectively transmits power supplied by a motor to the tray, thereby allowing the tray to be rotated at a uniform speed and to be driven even using a motor having a small load capacity.