A disconnection detecting method includes charging a capacitor by connecting a node of the capacitor to a first power source line supplied with a first power source potential, connecting the node of the capacitor to an input terminal, after the node of the capacitor is disconnected from the first power source line, and converting a first value on the node to a first digital data. The method further includes discharging the capacitor by connecting the node of the capacitor to a first power source line supplied with a second power source potential, after the node is disconnected from the input terminal, connecting the node of the capacitor to the input terminal, after the node of the capacitor is disconnected from the second power source line, and converting a second value on the node to a second digital data. The method further includes comparing the first digital data with the second digital data to determine whether a difference between the first and second digital data exists, and determining that the input terminal is abnormal when the difference exists.