PURPOSE: To allow only microwave to pass and to prevent gas from passing by forming a connection port to a gas piping at both edges of a joint body and at the same time mounting a dielectric so that the inside of the joint body may be blocked.

CONSTITUTION: In a gas insulation joint 22, connection ports 24 and 24a to a gas piping 1 are formed at both edges of a joint body 23 and a dielectric 25 is mounted so that the gas flow can be blocked and impedance can be adjusted by changing the thickness is mounted to the inside. Then, the gas insulation joint 22 is mounted to a microwave input tube 4a of microwave oven and gas heater 4. A microwave insulation joint 29 where a wire net for preventing microwave to pass and allowing only gas to pass is incorporated is mounted to the inside a gas input pipe 26 at the side of a gas burner 4b of a gas heater 27 and the microwave oven and gas heater 4, thus enabling the dielectric to block gas positively and hence preventing gas from being leaked at the down-stream side.