To reduce a passing loss even in the operation of one amplifier by reducing the passing loss by switching an open stab circuit at the time of combining input ports when the passing loss deteriorates.

An open stab circuit 16 is provided at a node B of 50Ω line 10 and 50Ω line 11. This open stab circuit 16 is composed of 50Ω line 12, switch 15 and 50Ω lines 13 and 14 to switch the connection through this switch 15. Then, when passing the output voltage of only one amplifier through a combiner, mismatching with an impedance conversion line 9 gets maximum and the passing loss is increased. Then, the switch 15 of open stab circuit 16 is changed over. Namely, since impedance matching can be made best by switching the open stab circuit 16, the passing loss can be suppressed small in the case of passing only the output voltage of one amplifier through the combiner when combining (n) pieces of ports.