A battery pack comprising: a battery cell; a pack main body having a top face, a bottom face, opposite side faces (11c,11d), a front face and a back face (I If), housing the battery cell; a plurality of terminals(23a, 23b, 23c) connected to the battery cell; a first concave portion and a second concave portion located on the front face; wherein a cross section of each of the first concave portion and the second concave portion, as viewed from the front face, forms a four sided shape having two parallel sides of unequal length, a third side substantially perpendicular to and connecting the two parallel sides and a fourth side connecting the two parallel sides opposite to the third side; wherein a shorter side of the parallel sides of each of the four sided shapes corresponds to one of the opposite side faces of the pack main body; wherein a longer side of the parallel sides of one of the four sided shapes faces a longer side of the parallel sides of the other one of the four sided shapes across the plurality of terminals; wherein the pack main body includes chamfered surfaces at corner portions formed by a bottom surface and opposite side faces and the chamfered surfaces correspond to the fourth sides of the four sided shapes; wherein two corners of the four sided shapes, which are formed by the fourth sides and the parallel sides, are curved; and wherein, as viewed from the bottom face, a depth of the first concave portion closer to the plurality of terminals is shallower than a depth of a second concave portion further from the plurality of terminals wherein, as viewed from the bottom face, the second concave portion comprises a wall on a longer side of the parallel sides. a battery cell; a pack main body having a top face, a bottom face, opposite side faces (11c,11d), a front face and a back face (I If), housing the battery cell; a plurality of terminals(23a, 23b, 23c) connected to the battery cell; a first concave portion and a second concave portion located on the front face; wherein a cross section of each of the first concave portion and the second concave portion, as viewed from the front face, forms a four sided shape having two parallel sides of unequal length, a third side substantially perpendicular to and connecting the two parallel sides and a fourth side connecting the two parallel sides opposite to the third side; wherein a shorter side of the parallel sides of each of the four sided shapes corresponds to one of the opposite side faces of the pack main body; wherein a longer side of the parallel sides of one of the four sided shapes faces a longer side of the parallel sides of the other one of the four sided shapes across the plurality of terminals; wherein the pack main body includes chamfered surfaces at corner portions formed by a bottom surface and opposite side faces and the chamfered surfaces correspond to the fourth sides of the four sided shapes; wherein two corners of the four sided shapes, which are formed by the fourth sides and the parallel sides, are curved; and wherein, as viewed from the bottom face, a depth of the first concave portion closer to the plurality of terminals is shallower than a depth of a second concave portion further from the plurality of terminals wherein, as viewed from the bottom face, the second concave portion comprises a wall on a longer side of the parallel sides.