PURPOSE: To provide an antenna horn cap which forcedly interrupts the transmission/reception of a radio wave from an antenna horn used for a satellite communication system and which can easily be installed at the tip of the antenna horn with one hand.

CONSTITUTION: The antenna horn cap is provided with a cap main body 2 where a restraining part 7 is formed at one end of a blocking board 6 on which a radio wave absorbing body 5 is stuck on the whole surface and a bending part 8 is formed on the other end of the blocking board 6, a pressing tool 3 which has a pressure part 11 and which is supported by the blocking board 6 so that it can move forwards and backwards, and a screw 4 screwed onto the bending part 8 so that it can move forwards and backwards in the same direction as the pressing tool 3. The pressure part 11 moves forwards by the restraint of the screw 4 and is pressurized to the outer peripheral part of the antenna horn on the opposite side of the restraining part 7 when the restraining part 7 is restrained to the outer peripheral part of the antenna horn and the screw is rotated in one direction.