To provide a unit capable of changing the voltage level in a state, where a signal line is connected by providing a switch circuit which changes over connected/non-connected states between a pull-up resistance or a pull-down resistance and a signal terminal or a constant voltage source and switching the presence or absence of the application of prescribed voltage to the signal terminal.

When electronic units 1 and 2 are not physically connected by a signal line 7, the signal terminal 11 of the first electronic unit 1 becomes a ground level GND. When a transistor 24 is not conducting even though the electronic units 1 and 2 are connected physically, the signal terminal 11 becomes a ground level GND. When the signal line 7 is fitted to the signal terminal 11 and a signal terminal 21, the electronic units 1 and 2 are connected physically, the transistor 24 is conducted and pull-up resistance 23 is connected to a power line 22, a level which is almost equal to the voltage level Vdd of the power line 22 appears to the signal terminal 11.