An assembled-battery system, a semiconductor circuit, and a diagnostic method enables appropriate self-diagnosis of a measuring unit. An output value (A-B) output from an analog-to-digital converter through power-supply lines, a cell-selection switch, and a level shifter is summed with an output value (B-VSS) obtained by a directly input reference voltage B being output from the analog-to-digital converter. When the summed value is considered equal the reference voltage A—the voltage VSS, it is diagnosed that an abnormality such as a breakdown has not occurred.