A microwave oscillator is effectively cooled and its output characteristic is stabilized, while the waterproof property of the microwave oscillator is secured. Additionally, the heat of the air heated as a result of cooling the microwave oscillator is discharged effectively to maintain the cooling effect. Furthermore, the cooling structure of the microwave oscillator is simplified and downsized and the cost of the arrangement is reduced. A structure includes an air blower member for blowing air to a microwave oscillator and a heat radiating/air circulating member airtightly connected to the terminating end of a microwave waveguide and a shield box so as to be able to cool the air introduced into the microwave waveguide after cooling the microwave oscillator as the air blower member is driven to by turn drive the air to flow from the terminating end toward the shield box. The air in the shield box and the microwave waveguide is enabled to circulate.