PURPOSE: To sufficiently amplify a signal sent out from a first device and whose amplitude is suppressed with a second device by using a power source with polarity opposite to the of a main power source by supplying the power source with polarity opposite to that of the main power source from the first device to the second device.

CONSTITUTION: When the signal intending to supply from a personal computer 1 to a device 3 is supplied to a signal sending circuit 7, the signal is superimposed on the negative power source of a sub power source 17. A superimposed signal is sent out from the emitter of a transistor 15 to a power source extraction circuit 9 and a signal extraction circuit 11. The signal supplied to the extraction circuit 9 is eliminated by separating from the negative power source on which the signal is superimposed. In such a way, negative potential (-VEE) can be obtained, and it is supplied to an amplifier circuit 13 as the power source on one side of the amplifier circuit 13. In other words, the signal sent out from the personal computer 1 can be amplified by source potential difference (5+VEE).