A signal line, which branches into four more from a connecting line on the input side via a branch point and has the branch signal lines including λ/4 transmission lines at their parts, and FETs respectively connected in shunt with the branch signal lines between connecting points on the output terminal sides as viewed from the λ/4 transmission lines provided in the branch signal lines and ground ends are provided on a semiconductor substrate. Connecting points of the FETs at the two branch signal lines are disposed with being spaced such a distance that isolation corresponding to the frequency of an RF signal reaches more than equal to 25 dB and less than or equal to 35 dB at the ends of these branch signal lines.