PURPOSE: To protect circuit elements from noise generated during live line replacement in a live line replacing device for substrate with a large current flowing.

CONSTITUTION: A live line replacing device 1 of a substrate is equipped with a wiring substrate 5 and a mother board 8 having a signal bus 10 which outputs power supply and receives logic signals. The wiring substrate 5 is equipped with a control IC 11 which operates on the power supply from the mother board 8 and controls the signal bus 10, a switch 17 for switching a control signal, and a NAND gate 15 which is able to prohibit the output of logic signal from the control IC 11 by means of the control signal. In this device, the control signal is controlled by the switch 17 while noise is generated during live line replacement, and the output of a logic signal or noise output from the NAND gate 15 can be prohibited.