In an identifier assignment system for assigning identifiers to respective voltage monitors to be connected to respective battery units, each measuring unit measures elapsed time information indicative of an elapsed time that has elapsed since a predetermined point of time after an activation of the corresponding voltage monitor based on power supplied from the corresponding battery unit connected to the corresponding voltage monitor. Each measuring unit is also configured to wirelessly transmit the elapsed time information about the corresponding voltage monitor. A comparison unit performs a comparison task of comparing the elapsed times of the respective voltage monitors with each other. An assignment unit is configured to assign unique identifiers to the respective voltage monitors in accordance with a result of the comparison task.