A switch group selectively outputs a signal input from IC terminals and a reference voltage. Another switch group selectively outputs a signal input from IC terminals and a reference voltage. A differential amplifier amplifies a differential voltage between a signal output from the switch group and a signal output from the another switch group. The switch group and the another switch group include the same number of switches. When to select any of signals input from the IC terminals in the switch group, a reference voltage is selected in the another switch group. When to select any of signals input from the IC terminals in the another switch group, a reference voltage is selected in the switch group.