When a monopole antenna is placed at one of the two focal points of an elliptical chamber and microwaves are radiated into the chamber, the object to be heated at the other focal position can be heated with high efficiency. Depending on the shape of the object to be heated, the field inside the chamber may be disturbed and reflected waves may be generated. Therefore, we installed a regulator called a stub tuner that can match the impedance in the chamber. This cancels the reflected waves in the chamber. Simulations show that 99.97% of the microwaves are absorbed by the object being heated.