To provide a binder device in which the pressure to an ankle is reduced as a boot is put on, in which an Achilles' tendon is protected, and in which bending action of the ankle can be naturally performed.

A down-directed aperture 10 is formed at a lower end center part of a high back 4 of a binding device to reduce pressure on a heel, a center part of a band material 15 of an ankle strap 6 to tighten a front surface part of an ankle part of a boot is branched upper and lower, leaf-like aperture 7 is formed between the branched band parts, and a swollen part is provided on the back surface side of the band member 15 to be positioned in the leaf-like aperture 7. A recessed part 23 is formed on the back surface of the swollen part, and a pad member 16 having a groove 24 is sewn along length of the swollen part to provide expansion effect in the width direction at the part of the branched band part, thereby ankle bending performance is improved.