A battery monitoring system for electric power tool includes a battery pack for electric power tool and an auxiliary power source. The battery pack has a battery and a monitoring circuit. The battery is provided with at least one battery cell. The monitoring circuit is operated by electric power supplied from the battery, and monitors a status of the battery. The auxiliary power source outputs electric power allowing the monitoring circuit to operate. The monitoring circuit is configured such that, when the monitoring circuit is unoperational with the electric power of the battery due to a decrease in a voltage of the battery, the monitoring circuit becomes operational by the electric power supplied from the auxiliary power source.