A battery monitoring system of flying capacitor type detects voltage difference between adjacent unit cells connected in series in battery pack. The system has the unit cells, an open and close section, a first and second capacitor, a first and second switching section, an ADC, and a microcomputer. The microcomputer instructs the open and close section to charge each of the first and second capacitors with each unit cell. After the first and second capacitors have been charged by the corresponding unit cells, the microcomputer instructs the first switching section to connect the first capacitor and the second capacitor in a reverse-polarity series connection. The microcomputer allows the second switching section to supply a voltage at both ends of the first and second capacitors connected in the reverse-polarity series connection, i.e. a voltage difference between the unit cells, to the ADC. The ADC supplies a conversion result to the microcomputer.