PURPOSE: To attain the automatic make/break of a power supply without using an exclusive circuit nor a control program, by closing a switching circuit only for a prescribed period of time after the signal on a transmission line is detected and securing the automatic application of the power supply.

CONSTITUTION: A master device 11 delivers continuously the polling signals (a) onto a transmission line 2 at a fixed time interval when the transmission of the sales data is requested to each slave device 14. The signal (a) is supplied to a power supply controller 17 via a branching device 13 and a signal extracting line 18. A signal detecting circuit 20 of the controller 17 delivers the pulse signals (c) of the prescribed short width synchronously with the input time points t0, t1... of the signals (a). A monostable circuit 21 receives the first signal (c) and has a rise to an H level from an L level at the time point t0 and is kept at the H level just for a prescribed period of time. A switching circuit 23 is closed when a control terminal is set at an H level at the point t0. Then an AC power supply (e) is supplied to each device 14 via a power supply line 22.