A battery pack (2; 602) includes: an outer case (12; 612); one or more battery cells (90a-90j); and a cell case (80), which is housed in the outer case (12; 612) and houses the battery cell(s) (90a-90j). The outer case (12; 612) has: an upper surface (wall) (14b), in which a terminal-opening part (22a) for exposing a terminal (102) is provided; a bottom wall (15e); and a plurality of side surfaces (walls) (15a-15d) extending upward from the bottom wall (15e). The battery cell(s) (90a-90j) is (are) disposed parallel to the bottom wall (15e). An opening (40a-40j) is provided in a specific side surface (15b, 15d), from among the plurality of side surfaces (15a-15d), that faces an end surface of the battery cell(s) (90a-90j) in a longitudinal direction.