The present invention includes a directional coupler that detects at least part of a reflected wave inside a waveguide. When an object to be heated is absent, a reflected wave is strong, because there is nothing to absorb a microwave. When the object is present, the reflected wave is weak, because the microwave is absorbed by the object. As a quantity of the object becomes greater, a larger amount of microwave is absorbed by the object, and thus the reflected wave is weakened. It is thus possible to determine the quantity of the object from a reflected wave detection amount detected by the directional coupler without using a detector that detects a load.