A battery case comprising: a case body in bottomed rectangular parallelopiped shape having a substantially rectangular 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: a pair of long side portions (43a, 44a) facing each other; a pair of short side portions (45a, 46a) located at both ends of the pair of long side portions (43a, 44a) and facing each other; R portions (47a - 47d) provided at four corners between the long side portions (43a, 44a) and the short side portions (45a, 46a); and steps (48a, 48b) protruding from the inner surfaces of the pair of short side portions (45a, 46a), each the R portions (47a - 47d) comprises a gradual change region (47c3), and in each the gradual change region (47c3), along each the R portions (47a - 47d), a shape of each the steps (48a, 48b) changes to be gradually close to a shape of each the inner surfaces of the long side portions (43a, 44a) toward the long side portions (43a, 44a), wherein the inner surfaces of the pair of long side portions (43a, 44a) facing each other are tapered surfaces inclined inward toward their lower portions, and in the gradual change region (47c3), the shape of each the steps (48a, 48b) changes to be gradually close to each the tapered surfaces toward the long side portions (43a, 44a), the battery case characterized by having a gap which is wider toward the upper edge of the opening (41a1), between a portion having each the tapered surfaces and the sealing plate, with the sealing plate attached to the opening (41a1). a case body in bottomed rectangular parallelopiped shape having a substantially rectangular 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: a pair of long side portions (43a, 44a) facing each other; a pair of short side portions (45a, 46a) located at both ends of the pair of long side portions (43a, 44a) and facing each other; R portions (47a - 47d) provided at four corners between the long side portions (43a, 44a) and the short side portions (45a, 46a); and steps (48a, 48b) protruding from the inner surfaces of the pair of short side portions (45a, 46a), a pair of long side portions (43a, 44a) facing each other; a pair of short side portions (45a, 46a) located at both ends of the pair of long side portions (43a, 44a) and facing each other; R portions (47a - 47d) provided at four corners between the long side portions (43a, 44a) and the short side portions (45a, 46a); and steps (48a, 48b) protruding from the inner surfaces of the pair of short side portions (45a, 46a), each the R portions (47a - 47d) comprises a gradual change region (47c3), and in each the gradual change region (47c3), along each the R portions (47a - 47d), a shape of each the steps (48a, 48b) changes to be gradually close to a shape of each the inner surfaces of the long side portions (43a, 44a) toward the long side portions (43a, 44a), wherein the inner surfaces of the pair of long side portions (43a, 44a) facing each other are tapered surfaces inclined inward toward their lower portions, and in the gradual change region (47c3), the shape of each the steps (48a, 48b) changes to be gradually close to each the tapered surfaces toward the long side portions (43a, 44a), the battery case characterized by having a gap which is wider toward the upper edge of the opening (41a1), between a portion having each the tapered surfaces and the sealing plate, with the sealing plate attached to the opening (41a1).