A contact diameter, which is formed by a cell holding portion and receiving-partitioning portions, is configured to be smaller than the diameter of a columnar outer peripheral surface of a battery cell. The cell holding portion comes into contact with the columnar outer peripheral surface of the battery cell so as to hold the columnar outer peripheral surface. The contact diameter formed by the cell holding portion and the receiving-partitioning portions increases when the battery cell is held, and the battery cell is pressed when the battery cell is held. A cell holder is provided with the receiving-partitioning portions that guide the battery cells to be inserted and housed.