A radiation element for radiating a microwave into the heating chamber at least one hollow dielectric member in which a gas is sealed, the hollow dielectric member having electrodes and a control unit having a plasma control unit for controlling the state of the hollow dielectric member and a current adjustment unit for adjusting a current to be applied to the electrodes of the hollow dielectric member under the control of the plasma control unit, the current adjustment unit being connected to the electrodes; and wherein the plasma control unit controls the state of the hollow dielectric member to put into one of states of: a plasma state in which the microwave is reflected by the gas; a plasma state in which the microwave is absorbed by the gas; and a gas state in which the microwave is allowed to be transmitted through the gas.