A balloon to be arranged on a medical catheter, comprising: a dilatable cylindrical portion that is formed with a birefringent polymer material, wherein a ratio of the number of orientation distributions calculated by dividing the number cos 2 Ø x of orientation distributions throughout a total length of the cylindrical portion in a circumferential direction (r) by the number cos 2 Ø z of orientation distributions throughout a total length of the cylindrical portion in an axial direction is less than 2, wherein cos 2 Ø x and cos 2 Ø z are derivable by the following equations cos 2 ̅ ∅ x + cos 2 ̅ ∅ y + cos 2 ̅ ∅ z = 1 cos 2 ̅ Ø x = n z + cos 2 ̅ Ø y cos 2 ̅ Ø z = n x + cos 2 ̅ Ø y 1 − n x − n z / 3 = cos 2 ̅ Ø y n z = Δ n length / Δ n * n x = Δ n axis / Δ n * Δ n length = nl − nd Δ n axis = nr − nd n 2 = nr 2 + nd 2 + nl 2 / 3 with cos 2 Ø y indicating the number of orientation distributions in the thickness direction (d) of the cylindrical portion, n r being the refractive index of the balloon material in the circumferential direction (r) of the cylindrical portion, n d being the refractive index of the balloon material in the thickness direction (d) of the cylindrical portion, n I being the refractive index of the balloon material in the longitudinal direction of the cylindrical portion, and Δn ∗ being the value of birefringence shown in the balloon material when molecules are oriented by 100%. a dilatable cylindrical portion that is formed with a birefringent polymer material, wherein a ratio of the number of orientation distributions calculated by dividing the number cos 2 Ø x of orientation distributions throughout a total length of the cylindrical portion in a circumferential direction (r) by the number cos 2 Ø z of orientation distributions throughout a total length of the cylindrical portion in an axial direction is less than 2, wherein cos 2 Ø x and cos 2 Ø z are derivable by the following equations cos 2 ̅ ∅ x + cos 2 ̅ ∅ y + cos 2 ̅ ∅ z = 1 cos 2 ̅ Ø x = n z + cos 2 ̅ Ø y cos 2 ̅ Ø z = n x + cos 2 ̅ Ø y 1 − n x − n z / 3 = cos 2 ̅ Ø y n z = Δ n length / Δ n * n x = Δ n axis / Δ n * Δ n length = nl − nd Δ n axis = nr − nd n 2 = nr 2 + nd 2 + nl 2 / 3 with cos 2 Ø y indicating the number of orientation distributions in the thickness direction (d) of the cylindrical portion, n r being the refractive index of the balloon material in the circumferential direction (r) of the cylindrical portion, n d being the refractive index of the balloon material in the thickness direction (d) of the cylindrical portion, n I being the refractive index of the balloon material in the longitudinal direction of the cylindrical portion, and cos 2 Ø y indicating the number of orientation distributions in the thickness direction (d) of the cylindrical portion, n r being the refractive index of the balloon material in the circumferential direction (r) of the cylindrical portion, n d being the refractive index of the balloon material in the thickness direction (d) of the cylindrical portion, n I being the refractive index of the balloon material in the longitudinal direction of the cylindrical portion, and Δn ∗ being the value of birefringence shown in the balloon material when molecules are oriented by 100%.