A high frequency signal switching unit includes a printed circuit board accommodated in the casing and having a ground layer formed on substantially one entire surface thereof. Input and output terminals are mounted on the casing, and a high frequency relay is mounted on the printed circuit board for switching between the input terminal and the output terminal. The high frequency relay has a plurality of relay contact terminals and a grounding terminal. A plurality of coaxial cables having respective internal and external conductors connect the input and output terminals to the relay contact terminals. The grounding terminal of the high frequency relay and the external conductors of the coaxial cables are connected to the ground layer of the printed circuit board, while each of the internal conductors of the coaxial cables is connected to one of the relay contact terminals of the high frequency relay.