An assist device includes a body mounting fixture, an actuator, an operation state detector, and a controller. The controller is configured to control drive of the actuator. The controller is configured to acquire an estimated posture of the wearer, which is estimated based on operation detection information detected by the operation state detector, when the actuator generates the assist torque. The controller is configured to determine whether or not the estimated posture is an unreasonable posture in which an excessive force is applied to a lower back portion.