PURPOSE: To prevent a switching control circuit from switching a device to the device, which is connected to an abnormal power supply part, in accordance with an erroneous switching command signal from the device having the abnormal power supply part by providing a power supply abnormality detector and an inhibiting circuit.

CONSTITUTION: In case of the abnormality of power supply where the power source voltage of a device #1 is lowered gradually, the device #1 transmits two signals of a switching command signal #1-CP and a switching request signal #1-CR to the external. At this time, an amplifier AMP-3 in a power supply abnormality detector #1-PD detects that the power source voltage of the device #1 is lower than a normal voltage VL1, and the terminal 2 of a gate circuit G4 is set to "1" by the output of this amplifier AMP-3, and therefore, the terminal 1 of an inhibiting circuit G3 is set to "1", and the switching command signal #1-CD applied to the terminal 2 cannot pass through the inhibiting circuit G3. As the result, the switching from a device #0 to the device #1 is not performed because the state of a flip flop D-FF is held as it is.