PURPOSE: To obtain the waveguide suitable for transmission and radiation of a large output microwave in which no multipactor discharge takes place by forming a gas plasma of a carbon compound comprising hydrogen and a gas in the inside of the waveguide through which an electromagnetic wave propagates and forming a carbon based thin film on the inner face by the chemical gas phase growing method.

CONSTITUTION: After the inside of the waveguide 7 is exhausted, a gas including a carbon compound comprising hydrogen and gas is injected to the waveguide from a gas supply system and a pressure in the waveguide 7 is kept constant and a plasma is generated in the waveguide 7 by providing a microwave into the waveguide. The carbon of the carbon compound separated by the stimulating action of the plasma 4 is vapor-depositted on copper plating 7a applied to the inner face of the stainless-steel made waveguide 7 to form a carbon group thin film 13 by the chemical gas phase growing method. A secondary electron emission factor of the carbon based thin film 13 is one or below, no multipactor discharge takes place and a microwave is stably transmitted even when a large output microwave is propagated by forming the carbon based thin film 13 to the inner side of the waveguide 7 in this way.