A power storage device comprising: an outer casing; and two or more battery units (51,52) housed in the outer casing, the battery units (51, 52) each including: a battery casing formed with a top casing having one or more first holes formed in a bottom surface portion thereof and a bottom casing having one or more second holes formed in a bottom surface portion thereof; a battery block group housed in the battery casing, a plurality of battery lines each formed with a plurality of battery cells arranged in a line are arranged in parallel in a direction substantially perpendicular to an extending direction of the battery lines (L1-L16), and are arranged like stacked straw bags; and a partition plate housed together with the battery block group in the battery casing, the partition plate being inserted between adjacent ones of the battery lines (L1-L16) facing each other, one or more first protrusions being formed on an upper surface of the partition plate, one or more second protrusions being formed on a lower surface of the partition plate, the one or more first protrusions being engaged with the one or more first holes, the one or more second protrusions being engaged with the one or more second holes, characterized by a joining member electrically connecting the battery cells and housed in the battery casing and electrically joined to all terminal surfaces of the battery cells constituting at least one of the battery lines (L1-L16), a first board housed in the outer casing and secured to one wall surface of the battery casing, a protruding end portion (91b1) of the joining member being joined to the first board, the protruding end portion (91b1) protruding in the battery line (L1-L16) extending direction, a circuit including at least a voltage control circuit being mounted on the first board, and a second board and a third board housed in the outer casing, the second board and the third board being formed as structures independent of the first board, a circuit including at least a current breaking element being mounted on the second board, a circuit including at least an output terminal being mounted on the third board. an outer casing; and two or more battery units (51,52) housed in the outer casing, the battery units (51, 52) each including: a battery casing formed with a top casing having one or more first holes formed in a bottom surface portion thereof and a bottom casing having one or more second holes formed in a bottom surface portion thereof; a battery block group housed in the battery casing, a plurality of battery lines each formed with a plurality of battery cells arranged in a line are arranged in parallel in a direction substantially perpendicular to an extending direction of the battery lines (L1-L16), and are arranged like stacked straw bags; and a partition plate housed together with the battery block group in the battery casing, the partition plate being inserted between adjacent ones of the battery lines (L1-L16) facing each other, one or more first protrusions being formed on an upper surface of the partition plate, one or more second protrusions being formed on a lower surface of the partition plate, the one or more first protrusions being engaged with the one or more first holes, the one or more second protrusions being engaged with the one or more second holes, a battery casing formed with a top casing having one or more first holes formed in a bottom surface portion thereof and a bottom casing having one or more second holes formed in a bottom surface portion thereof; a battery block group housed in the battery casing, a plurality of battery lines each formed with a plurality of battery cells arranged in a line are arranged in parallel in a direction substantially perpendicular to an extending direction of the battery lines (L1-L16), and are arranged like stacked straw bags; and a partition plate housed together with the battery block group in the battery casing, the partition plate being inserted between adjacent ones of the battery lines (L1-L16) facing each other, one or more first protrusions being formed on an upper surface of the partition plate, one or more second protrusions being formed on a lower surface of the partition plate, the one or more first protrusions being engaged with the one or more first holes, the one or more second protrusions being engaged with the one or more second holes, characterized by a joining member electrically connecting the battery cells and housed in the battery casing and electrically joined to all terminal surfaces of the battery cells constituting at least one of the battery lines (L1-L16), a first board housed in the outer casing and secured to one wall surface of the battery casing, a protruding end portion (91b1) of the joining member being joined to the first board, the protruding end portion (91b1) protruding in the battery line (L1-L16) extending direction, a circuit including at least a voltage control circuit being mounted on the first board, and a second board and a third board housed in the outer casing, the second board and the third board being formed as structures independent of the first board, a circuit including at least a current breaking element being mounted on the second board, a circuit including at least an output terminal being mounted on the third board.