dc to an ac and outputs the ac to the power switching circuit 3 the power switching circuit 3 selectively outputs the ac of the emi filter 2 or the ac of the dc ac inverter 5 the signal generator circuit 7 generates a ring signal and outputs it to line switching circuit 9 under control of the control circuit 6 when a power failure occurs namely when the power supply from the commercial power stops the emi filter 8 is connected to a telephone line and the line switching circuit 9 and filters a noise in the telephone line the line switching circuit 9 selectively outputs the ring signal of the signal generator circuit 7 or an output of the emi filter for line 8 the control circuit 6 detects a power failure or an abnormality in the commercial power controls the signal generator circuit 7 and the switchings performed in the power switching circuit 3 and the line switching circuit 9 in detail when a power failure occurs or the control circuit 6 detects an abnormality in the commercial power the control circuit 6 outputs a switch control signal to the power switching circuit 3 and the line switching circuit 9 the power switching circuit 3 switches to the output of the battery 4 in response to the switch control signal the line switching circuit 9 switches from the normal telephone line to the output of the signal generator circuit 7 in response to the switch control signal the computer 10 includes an ac dc power supply circuit 11 a central processing unit cpu 12 and a modem circuit 13 the ac dc power supply circuit 11 receives.