A semiconductor circuit, battery monitoring system, diagnostic program and diagnosis method are provided enabling appropriate self-diagnosis of a measurement unit. An output value (A−B) output through respective power supply lines V (Vn, Vn−1), a cell selector switch, and a level shift circuit from an AD converter and an output value (B) of a directly input reference voltage B output from the AD converter are summed together. Diagnosis is made that no abnormality such as a defect has occurred when the summed value is substantially the same as a reference voltage A.