A battery pack to be inserted into and attached to a power tool, comprising: a case; a plurality of battery cells accommodated inside the case and including a horizontally placed battery cell (31, 32) and a vertically placed battery cell; and electrode terminals (50A, 50B) connected to the battery cells and provided on the case so as to be connectable to a tool-side electrode terminal, wherein the case includes: a horizontally-elongated case main portion; and a case sub portion protruding from the case main portion and to be inserted into the power tool, characterised in that the case is L-shaped in a side view and the case sub portion is upwardly protruding from the case main portion in a direction orthogonal to the case main portion, wherein the case main portion accommodates the horizontally placed battery cell (31, 32) and the case sub portion accommodates the vertically placed battery cell, a step portion is formed on the upper surface of the case main portion, and a proximal side of the case main portion is formed higher than a distal side of the case main portion, with the step portion as a boundary, and wherein slits (211A, 211B) are formed in the step portion along the arrangement direction of laterally placed battery cells, and the electrode terminals (50A, 50B) are provided in the slits and are located on the upper surface of the case main portion at a side of the case sub portion and more on a side of the case main portion than an imaginary line connecting a protruding end of the case sub portion and an upper surface end of the case main portion located so that the electrode terminals are interposed between the case main portion and the case sub portion. a case; a plurality of battery cells accommodated inside the case and including a horizontally placed battery cell (31, 32) and a vertically placed battery cell; and electrode terminals (50A, 50B) connected to the battery cells and provided on the case so as to be connectable to a tool-side electrode terminal, wherein the case includes: a horizontally-elongated case main portion; and a case sub portion protruding from the case main portion and to be inserted into the power tool, a horizontally-elongated case main portion; and a case sub portion protruding from the case main portion and to be inserted into the power tool, characterised in that the case is L-shaped in a side view and the case sub portion is upwardly protruding from the case main portion in a direction orthogonal to the case main portion, wherein the case main portion accommodates the horizontally placed battery cell (31, 32) and the case sub portion accommodates the vertically placed battery cell, a step portion is formed on the upper surface of the case main portion, and a proximal side of the case main portion is formed higher than a distal side of the case main portion, with the step portion as a boundary, and wherein slits (211A, 211B) are formed in the step portion along the arrangement direction of laterally placed battery cells, and the electrode terminals (50A, 50B) are provided in the slits and are located on the upper surface of the case main portion at a side of the case sub portion and more on a side of the case main portion than an imaginary line connecting a protruding end of the case sub portion and an upper surface end of the case main portion located so that the electrode terminals are interposed between the case main portion and the case sub portion.