To provide a waterproof device for attachment of liquid-cooled magnetron, capable of mounting a liquid-cooled magnetron on the upper surface of a waveguide launcher easy to mount while interrupting water flowing from the liquid-cooled magnetron by dew condensation.

An elastic member 40 having a frame diameter capable of enclosing a gasket 26 is provided around an antenna dome 13 inserted to an antenna hole 23 of the waveguide launcher from above, so that even if water flowing by dew condensation of the liquid-cooled magnetron 10 falls onto the upper surface of the waveguide launcher 22, the water is interrupted by the elastic member 30 to prevent intrusion thereof into the launcher 22 through the gasket 26.