An in-vehicle power storage device that discharges a battery in a case where collision of a vehicle is detected or predicted, and stops discharging the battery when an over-discharged state has not been reached, and a control method thereof. The in-vehicle power storage device includes: a battery mounted in a vehicle; charge-state detection means for detecting a charge state of the battery; and a discharging load for discharging the battery, and a control device which includes: battery charge-state detection means for detecting a charge state; a battery discharger means for discharging the battery using the load if collision of the vehicle is detected or predicted; and discharge stopping means to stop discharging the battery before the battery becomes over-discharged.