Abstract: This invention relates to a microwave oven which can generate and transfer maximum microwave power to food being cooked irrespective of the weight and size of that food being cooked, thereby shortening cooking time and improving the efficiency of cooking. The microwave oven includes a sensor for sensing the food load on a turntable in a heating chamber, a microcomputer for controlling the oven in response to key signals input by the user and in response to the load sensor, a power supply for supplying power to a magnetron to operate the magnetron under the control of the microcomputer, an impedance matching device for matching impedance by moving a reflector in a waveguide fixed to the heating chamber, a match driver for controlling the impedance matching device under the control of the microcomputer, a variable vane mounted in an opening of the heating chamber, and a variable vane driving means for rotating the variable vane under the control of the microcomputer.