Battery pack for electric power tool

A contact diameter, which is formed by a cell holding portion (63) and receiving-partitioning portions (61), is configured to be smaller than the diameter of a columnar outer peripheral surface (330) of a battery cell (33). The cell holding portion (63) comes into contact with the columnar outer peripheral surface (330) of the battery cell (33) so as to hold the columnar outer peripheral surface (330). The contact diameter formed by the cell holding portion (63) and the receiving-partitioning portions (61) increases when the battery cell (33) is held, and the battery cell (33) is pressed when the battery cell (33) is held. A cell holder (50) is provided with the receiving-partitioning portions (61) that guide the battery cells (33) to be inserted and housed.