A battery case comprising: a case body in bottomed rectangular parallelopiped shape having an opening (41a1) in one side surface facing a bottom surface; and a substantially rectangular sealing plate attached to the opening (41a1) and having a shape corresponding to an upper edge of the opening (41a1), wherein the opening (41a1) of the case body comprises steps (48a, 48b) protruding inward, on inner surfaces of a pair of short sides (45a, 46a) facing each other, the sealing plate which is a plate fitted into the opening (41a1) comprises: a pair of long side portions (91, 92) facing each other; a pair of short side portions (93, 94) located at both ends of the pair of long side portions (91, 92) and facing each other; and R portions (95 - 98) provided at four corners between the long side portions (91, 92) and the short side portions (93, 94), an edge of a lower surface (41b2) of the sealing plate is chamfered, characterized in that a chamfering amount of the pair of short side portions (93, 94) is greater than a chamfering amount of the pair of long side portions (91, 92), and each of the R portions (95 - 98) comprises a first region, a second region and a gradual change region, the first region is a region adjacent to one of the short side portions (93, 94), the second region is a region adj acent to one of the long side portions (91, 92), and the gradual change region is provided between the first region and the second region, wherein: a chamfering amount of the first region is equal to the chamfering amount of the short side portions (93, 94), a chamfering amount of the second region is equal to the chamfering amount of the long side portions (91, 92), and in the gradual change region, a chamfering amount gradually decreases from a first end on a side of the short side portions (93, 94) toward a second end on a side of the long side portions (91, 92), the first end is an endpoint of the first region, and the second end is a starting point of the second region. a case body in bottomed rectangular parallelopiped shape having an opening (41a1) in one side surface facing a bottom surface; and a substantially rectangular sealing plate attached to the opening (41a1) and having a shape corresponding to an upper edge of the opening (41a1), wherein the opening (41a1) of the case body comprises steps (48a, 48b) protruding inward, on inner surfaces of a pair of short sides (45a, 46a) facing each other, the sealing plate which is a plate fitted into the opening (41a1) comprises: a pair of long side portions (91, 92) facing each other; a pair of short side portions (93, 94) located at both ends of the pair of long side portions (91, 92) and facing each other; and R portions (95 - 98) provided at four corners between the long side portions (91, 92) and the short side portions (93, 94), a pair of long side portions (91, 92) facing each other; a pair of short side portions (93, 94) located at both ends of the pair of long side portions (91, 92) and facing each other; and R portions (95 - 98) provided at four corners between the long side portions (91, 92) and the short side portions (93, 94), an edge of a lower surface (41b2) of the sealing plate is chamfered, characterized in that a chamfering amount of the pair of short side portions (93, 94) is greater than a chamfering amount of the pair of long side portions (91, 92), and each of the R portions (95 - 98) comprises a first region, a second region and a gradual change region, the first region is a region adjacent to one of the short side portions (93, 94), the second region is a region adj acent to one of the long side portions (91, 92), and the gradual change region is provided between the first region and the second region, the first region is a region adjacent to one of the short side portions (93, 94), the second region is a region adj acent to one of the long side portions (91, 92), and the gradual change region is provided between the first region and the second region, wherein: a chamfering amount of the first region is equal to the chamfering amount of the short side portions (93, 94), a chamfering amount of the second region is equal to the chamfering amount of the long side portions (91, 92), and in the gradual change region, a chamfering amount gradually decreases from a first end on a side of the short side portions (93, 94) toward a second end on a side of the long side portions (91, 92), the first end is an endpoint of the first region, and the second end is a starting point of the second region. a chamfering amount of the first region is equal to the chamfering amount of the short side portions (93, 94), a chamfering amount of the second region is equal to the chamfering amount of the long side portions (91, 92), and in the gradual change region, a chamfering amount gradually decreases from a first end on a side of the short side portions (93, 94) toward a second end on a side of the long side portions (91, 92), the first end is an endpoint of the first region, and the second end is a starting point of the second region. the first end is an endpoint of the first region, and the second end is a starting point of the second region.