A low temperature high-frequency amplifying apparatus which includes an amplifying circuit and a cold stage for cooling the amplifying circuit. The amplifying circuit and the cold stage are accommodated in a thermally insulated container. An input connector of the thermally insulated container, which is exposed to the outside, is connected with an input portion of the amplifying circuit via an input cable, and an output portion of the amplifier is connected with an output connector of the thermally insulated container via an output cable. The signal transmission loss of the input cable is lower than the signal transmission loss of the output cable, and the amount of heat penetration from the output cable is less than the amount of heat penetration from the input cable.