A vehicle power supply system comprises a battery module that is constituted with a plurality of serially connected battery cell groups each include a plurality of serially connected battery cells, a plurality of battery cell control devices each disposed in correspondence to the plurality of battery cell groups, and a signal transmission path through which signals that are input to or output from the plurality of battery cell control devices are transmitted. Each of the plurality of battery cell control devices comprises a voltage measurement circuit that measures terminal voltages at the individual battery cells in the corresponding battery cell group, an abnormality diagnosis circuit that diagnoses an abnormality in the battery cell control device, a timing control circuit that outputs a signal for instructing measurement phase during which the terminalvoltage ismeasured, and, in synchronization with output of the measurement phase instruction signal, outputs a signal for instructing diagnosis phase during which diagnosis is executed by the abnormality diagnosis circuit, and a communication circuit that outputs a signal indicating the terminal voltage measured by the voltage measurement circuit and a signal based upon a diagnosis result by the abnormality diagnosis circuit, to the signal transmission path. The abnormality diagnosis circuit diagnoses an abnormality in the battery cell control device in synchronization with the terminal voltage measurement based upon the diagnosis phase instruction signal.