PURPOSE: To provide a bias circuit for transistor power amplifier which reduces the ripple type impact given to a power line when the output is discontinued for maintenance, etc.

CONSTITUTION: A transistor power amplifier 1 contains a transistor power amplifier 5 and a bias circuit 11. The circuit 11 contains a bias changeover switch 13 which selectively outputs the bias voltage that cuts off the amplifier 5 or the bias voltage that secures the normal operation of the amplifier 5 and a time constant circuit 15 which smoothly changes the output signal received from the switch 13 and supplies it to the amplifier 5. When the output of the amplifier 5 is going to be discontinued for maintenance, etc., the bias voltage is gradually changed by the circuit 15 even if the transmission/cut-off changeover is carried out by the switch 13. Thus the output of the amplifier 5 is also gradually changed.