PURPOSE: To prevent the transmission of a disturbance to a signal bus when an electronic circuit package is loaded into and unloaded out of a device by separating the circuit package from a signal bus in terms of signals in the package loading/unloading state.

CONSTITUTION: The contact of a relay contact circuit 5 is closed after an electronic circuit package 7 is loaded into a device, a power bus 102 and an ground bus 103 are connected to a power line (b) and an ground line (c) respectively, and then a switch circuit 6 is closed. In other words, the circuit 6 is kept in an OFF state at first when the package 7 is loaded into the device and then turned on after the package 7 is loaded in the device. As a result, a current flows to a relay circuit 4 and the circuit 6 is closed. When the package 7 is unloaded out of the device, the circuit 6 is turned off and the circuit 5 is turned on. Then the package 7 is unloaded out of the device. Thus the circuit 6 attains the operation of the device at its front side with the package 7 kept in the device.