PURPOSE: To shift 0V electric potential of a power supply road to an optional position on the power supply road by making output impedance variable.

CONSTITUTION: Power supply from power supply devices 1a and 1b simultaneously stars, and when DC is supplied to load resistance 4 (RA) and 5 (RB), electric potential of a branch device 7 is 0(V) against the earth, as long as the relation of (rA/rB)=(RA/RB) is available. Under the state, a relay contact 8 operates, and the contact is switched from a to b. An end power supply from a power supply device 1c can be done. When power supply from the power supply devices 1a and 1b is stopped, stop the power supply from the power supply device 1c before stopping the power supply from the power supply devices 1a and 1b, and if output impedance 3a (rA) is changed for impedance r, the branch device 7 releases the contact while keeping the electric potential of 0V, and the contact can be switched from b to a.