A battery pack (30; 30A) that is mountable on a power tool and on a charger, comprising: a case (31; 31A) constituting an outer shell; a plurality of columnar battery cells (72, 721-723; 72a, 72b) each having a longitudinal direction, the battery cells being disposed in an interior of the case (31; 31A) such that the battery cells (72, 721-723; 72a, 72b) are aligned and the longitudinal directions thereof are disposed in parallel; and a first ventilation hole and second ventilation holes, each of which are formed such that they permit fluid communication between the interior and an exterior of the case (31; 31A); wherein: a ventilation path (55, 56, Q1-Q3, 93-96) is defined in the interior of the case (31; 31A) such that the first ventilation hole serves as an inlet thereof and the second ventilation holes serve as outlets thereof; a plurality of longitudinal-direction passageways (Q1, Q3, 96), wherethrough a plurality of cooling drafts (F1, F2) respectively flow along the longitudinal directions of the battery cells (72, 721-723; 72a, 72b), is provided in the ventilation path, each longitudinal-direction passageway being arranged in the direction that the battery cells (72, 721-723; 72a, 72b) are disposed in parallel; and a longitudinal extension of each of the longitudinal-direction passageways (Q1, Q3, 96) extends in parallel to the longitudinal directions of the battery cells (72, 721-723; 72a, 72b), characterized in that at least one fixing wall provided on the inner bottom surface of the case (31; 31A) and contacting outer-circumferential surfaces of the battery cells (72, 721-723; 72b); wherein the at least one fixing wall isolates at least a portion (Q1-Q3) of the ventilation path from electrode-end surfaces (73, 731-734) of the battery cells (72, 721-723; 72b). a case (31; 31A) constituting an outer shell; a plurality of columnar battery cells (72, 721-723; 72a, 72b) each having a longitudinal direction, the battery cells being disposed in an interior of the case (31; 31A) such that the battery cells (72, 721-723; 72a, 72b) are aligned and the longitudinal directions thereof are disposed in parallel; and a first ventilation hole and second ventilation holes, each of which are formed such that they permit fluid communication between the interior and an exterior of the case (31; 31A); a ventilation path (55, 56, Q1-Q3, 93-96) is defined in the interior of the case (31; 31A) such that the first ventilation hole serves as an inlet thereof and the second ventilation holes serve as outlets thereof; a plurality of longitudinal-direction passageways (Q1, Q3, 96), wherethrough a plurality of cooling drafts (F1, F2) respectively flow along the longitudinal directions of the battery cells (72, 721-723; 72a, 72b), is provided in the ventilation path, each longitudinal-direction passageway being arranged in the direction that the battery cells (72, 721-723; 72a, 72b) are disposed in parallel; and a longitudinal extension of each of the longitudinal-direction passageways (Q1, Q3, 96) extends in parallel to the longitudinal directions of the battery cells (72, 721-723; 72a, 72b), characterized in that at least one fixing wall provided on the inner bottom surface of the case (31; 31A) and contacting outer-circumferential surfaces of the battery cells (72, 721-723; 72b); wherein the at least one fixing wall isolates at least a portion (Q1-Q3) of the ventilation path from electrode-end surfaces (73, 731-734) of the battery cells (72, 721-723; 72b).