PURPOSE: To prevent the burning of a closing coil by opening a lock contact by the detecting signal of a current detector even when a transfer contact is welded.

CONSTITUTION: When a transfer contact 5 cannot be opened, large currents are made to continuously flow through a circuit composed of a power supply, a closing contact, a current detector, a closing coil, the transfer contact and a lock contact. The current detector 4 detects currents made to flow through the coil at that time, information is transmitted over a microprocessor 6, and a command opening the lock contact 8 is output from the microprocessor 6 when the time when large currents are made to flow is made longer than a fixed time. When the lock contact 8 is opened, currents are made to flow through a series circuit consisting of the power supply, the closing contact, the current detector, the closing coil and a holding coil, currents determined by coil resistance made up of the closing coil and the holding coil are made to flow, and the state of closing is maintained in a section switch.