A series elastic actuator includes a gear, a mounting recess formed in the gear, an elastic body provided in the mounting recess, an output body, a through-hole penetrating the gear, a shaft passing through the through-hole, a pair of bearings to radially support the shaft, and a pair of bearing mounting recesses to receive the pair of bearings. The gear is configured to rotate by a rotational power source. A rotation force of the gear is transmitted to the gear by the elastic body. The through-hole may extend in an axial direction, and the pair of bearings may be spaced apart from each other in the axial direction. The pair of bearing mounting recesses may be formed at opposite ends of the through-hole.