The high-frequency circuit cooling apparatus comprises a package container 14 for housing a high-frequency circuit, a tank 16 for storing a gas to be introduced into the package container 14, a cold head 12 for cooling the package container 14 and the tank 16, pipes 24, 26 connected to the tank 16, for supplying the gas into the tank 16, pipes 18, 22 detachably connected between the tank 16 and the package container 14, for introducing the gas in the tank 16 into the package container 14, and pipes 34, 36 detachably connected to the package container 14, for discharging the gas in the package container 14.