A flat foot orthosis has two bodies (10a, 10b, 10c, 10d, 10e), and each has a wearing space, a foot portion (11), an ankle portion (12), a shank portion (14), a substrate (17), and a first elastomer (18). The substrate (17) and the first elastomer (18) are disposed adjacent to each other, and surround the wearing space. The substrate (17) extends from the foot portion (11) to the shank portion (14) through the ankle portion (14). The first elastomer (18) extends from an instep (112) at a cuboid bone (A) of a wearer (30) through a lateral side (114), a sole (112), and an arch (114) of the foot portion (11) in order, wraps over a navicular bone (B), and extends outwardly combining upwardly through the ankle portion (12) at a talus bone (C) toward the shank portion (14).
