To provide a plastic ankle foot orthosis freely adjusting the suspension of a foot bottom plate part.

The orthosis body is formed of the foot bottom plate part 2 and rear face support parts 3 erecting continuously therefrom, the rear face support parts 3 are defined as open type divided rear face support parts 8 formed by being divided into right/left by a vertically halving groove 4 extending from a heel receiving notch 6 to the upper end, the groove interval (s) of the vertically halving groove is set by fastening a wearing/fastening belt 9 to be attached to the both calf receiving parts 7 of the divided rear face support parts 8, the desired both calf receiving parts 7a are so constituted as to be fitted to the shape of the leg calf part, and an instep fastening belt 10 projecting from the bottom face to the both sides is freely detachably engaged with fixed hook-and-loop fasteners on the both sides of the foot bottom plate part 2 to determine the projecting angle and fastened corresponding to the shape of the foot instep.