PURPOSE: To decrease the number of switching terminals and the number of display device driving circuits by driving a changeover switch group with a constant current, inputting this terminal voltage to one of plural comparators, giving a prescribed reference voltage to the other input and using the combination of outputs of the comparators as a switching signal.

CONSTITUTION: A resistor R1 of the changeover switch group 10 is set as 1kohm and an R2 as 2kohms, and a current of 1mA is fed from a constant current source 32. A voltage at a terminal T2 is changed in the range of (1+Vf)∼(n+ Vf)V depending on the selected switch, where Vf is a forward voltage of an LED. On the other hand, in selecting resistors R1'∼Rn' respectively as 1kohm and a current I2 as 1mA and also setting the forward voltage of a diode 33 nearly to that of the LED, a voltage in the range of (1+Vf)∼(n+Vf)V is set as a reference voltage to comparators COM1∼COMn. Thus, the output of the comparators COM1∼COMn is changed depending on the selection of switches SW1 SWn, then the selected switch, i.e., the selected channel is known from the said output.