To provide a wideband amplifier, wherein its sub-board can be replaced without causing instantaneous interrupt of transmission signals.

A main board 20 contains amplifier transistors in addition to an input terminal 22, an output terminal 23, a high-frequency switching circuit 24, etc. Then a sub-board 30 can be attached to and detached from the board 20 via connectors 41 and 42 having three pins or more for control signals. The sub-board 30 includes a waveform shaping circuit 31 of BON, EQ, ATT, etc., and also a control circuit 32 which controls the circuit 24. The circuit 32 normally keeps the circuit 24 in an off-state and then turns on the circuit 24 when the sub-board 30 is detached from the board 20.