A balloon catheter (1, 2, 3A, 3B, 4A, 4B, 5A, 5B, 6A, 6B, 7, 8) comprising: a balloon (10, 10a, 10b, 10c, 10d, 10e, 10f, 10g); and an outer shaft (20, 20a, 20b, 20c, 20d, 20e, 20f, 20g) fixed to a proximal end of the balloon (10, 10a, 10b, 10c, 10d, 10e, 10f, 10g), characterised in that: each of the proximal end of the balloon (10, 10a, 10b, 10c, 10d, 10e, 10f, 10g) and a distal end of the outer shaft (20, 20a, 20b, 20c, 20d, 20e, 20f, 20g) has an inner projection (72, 72a, 82, 82a) extending in an axis direction, an outer projection (70, 70a, 80, 80a) extending in the axis direction, and a gap (71, 71a, 71b, 71c, 81, 81a, 81b, 81c) formed between the inner projection (72, 72a, 82, 82a) and the outer projection (70, 70a, 80, 80a); and i) the inner projection (72, 72a) of the balloon (10, 10a, 10c, 10d, 10f, 10g) is disposed in the gap (81, 81a, 81b, 81c) of the outer shaft (20, 20a, 20b, 20d, 20f, 20g) and the outer projection (80, 80a) of the outer shaft (20, 20a, 20b, 20d, 20f, 20g) is disposed in the gap (71, 71a, 71c) of the balloon (10, 10a, 10c, 10d, 10f, 10g), or ii) the outer projection (70, 70a) of the balloon (10, 10b, 10c, 10e, 10f, 10g) is disposed in the gap (81, 81a, 81b, 81c) of the outer shaft (20, 20a, 20b, 20e, 20f, 20g) and the inner projection (82, 82a) of the outer shaft (20, 20a, 20c, 20e, 20f, 20g) is disposed in the gap (71, 71a, 71b, 71c) of the balloon (10, 10b, 10c, 10e, 10f, 10g). a balloon (10, 10a, 10b, 10c, 10d, 10e, 10f, 10g); and an outer shaft (20, 20a, 20b, 20c, 20d, 20e, 20f, 20g) fixed to a proximal end of the balloon (10, 10a, 10b, 10c, 10d, 10e, 10f, 10g), characterised in that: each of the proximal end of the balloon (10, 10a, 10b, 10c, 10d, 10e, 10f, 10g) and a distal end of the outer shaft (20, 20a, 20b, 20c, 20d, 20e, 20f, 20g) has an inner projection (72, 72a, 82, 82a) extending in an axis direction, an outer projection (70, 70a, 80, 80a) extending in the axis direction, and a gap (71, 71a, 71b, 71c, 81, 81a, 81b, 81c) formed between the inner projection (72, 72a, 82, 82a) and the outer projection (70, 70a, 80, 80a); and i) the inner projection (72, 72a) of the balloon (10, 10a, 10c, 10d, 10f, 10g) is disposed in the gap (81, 81a, 81b, 81c) of the outer shaft (20, 20a, 20b, 20d, 20f, 20g) and the outer projection (80, 80a) of the outer shaft (20, 20a, 20b, 20d, 20f, 20g) is disposed in the gap (71, 71a, 71c) of the balloon (10, 10a, 10c, 10d, 10f, 10g), or ii) the outer projection (70, 70a) of the balloon (10, 10b, 10c, 10e, 10f, 10g) is disposed in the gap (81, 81a, 81b, 81c) of the outer shaft (20, 20a, 20b, 20e, 20f, 20g) and the inner projection (82, 82a) of the outer shaft (20, 20a, 20c, 20e, 20f, 20g) is disposed in the gap (71, 71a, 71b, 71c) of the balloon (10, 10b, 10c, 10e, 10f, 10g). i) the inner projection (72, 72a) of the balloon (10, 10a, 10c, 10d, 10f, 10g) is disposed in the gap (81, 81a, 81b, 81c) of the outer shaft (20, 20a, 20b, 20d, 20f, 20g) and the outer projection (80, 80a) of the outer shaft (20, 20a, 20b, 20d, 20f, 20g) is disposed in the gap (71, 71a, 71c) of the balloon (10, 10a, 10c, 10d, 10f, 10g), or ii) the outer projection (70, 70a) of the balloon (10, 10b, 10c, 10e, 10f, 10g) is disposed in the gap (81, 81a, 81b, 81c) of the outer shaft (20, 20a, 20b, 20e, 20f, 20g) and the inner projection (82, 82a) of the outer shaft (20, 20a, 20c, 20e, 20f, 20g) is disposed in the gap (71, 71a, 71b, 71c) of the balloon (10, 10b, 10c, 10e, 10f, 10g).