Commercial microwave oven including a main cabinet for enclosing and protecting various components, having air inlet holes, a base frame under the main cabinet having a box form with an opened top and a front part with a plurality of air outlet holes, a bottom plate on the base frame, an inner case in a space above one side part of the bottom plate, the inner case having a cooking chamber formed therein, a magnetron on a lower part of an outside of the inner case, a transformer for regulating a voltage provided to the magnetron, a waveguide on an underside of the bottom plate, a fan in a rear of outside of the inner case, and a flow guide for guiding air passed through the magnetron toward air outlet holes, thereby cooling components effectively, reducing a volume, and cost of the product.