The present invention relates to a microwave oven. The microwave oven of the present invention comprises a cavity assembly which defines a cooking chamber therein and functions as a framework of the microwave oven, an outer casing which includes a top portion and side portions formed ar both ends of the top portion to enclose the cavity assembly and interior parts and is provided with convexo-concave reinforcements at the top portion and at least one side portion, a door of which one side is connected to the cavity assembly to be a pivot center and which selectively causes the cooking chamber to be open and close, and a back plate which defines a rear face of the cavity assembly and is provided with a convexo-concave reinforcement formed along at least one edge portion thereof. According to the present invention constructed as such, there is an advantage in that external rigidity of the microwave oven is increased, whereby deformation of the microwave oven due to an impact or operating force can be minimized.