A power storage device includes an outer casing and two battery units housed in the outer casing. Each battery unit includes a battery casing formed with a top casing and a bottom casing. The battery casing houses a battery block group in which battery lines are arranged in parallel in a direction substantially perpendicular to the battery line extending direction, and are arranged like stacked straw bags. The battery casing also houses partition plates that are inserted between adjacent battery lines facing one another, and are secured between the top casing and the bottom casing.