PURPOSE: To supply a power supply voltage through a communication line and to normally drive a control means even when the power supply voltage supplied through a power supply line is lowered.

CONSTITUTION: Terminal equipments 2-4 provided in this controller 1 are connected by one communication line 5 and slave terminal equipments 3 and 4 send out response signals for data transmission request signals from a master terminal equipment 2 corresponding to predetermined priority order. The power supply voltage from a power unit 7 is supplied through the power supply line 6 to the terminal equipments 2-4 and the power supply voltage supplied to the master terminal equipment 2 is supplied through the communication line 5 to the slave terminal equipments 3 and 4. The power supply voltage supplied through the communication line 5 to the slave terminal equipment 3 is waveform-shaped in a constant voltage circuit 20 provided in the slave terminal equipment 3 and supplied to an arithmetic processing circuit 16.