A leg assist device having an abnormality management procedure which appropriately adapts to an abnormal situation is provided. The leg assist device is provided with a leg attachment and a controller. The leg attachment comprises upper and a lower links connected with a rotary joint, and an actuator. The upper link is to be attached to the upper leg of the user. The lower link is to be attached to the lower leg of the user. The actuator swings the lower link relative to the upper link. The controller outputs the commands so that the swing angle of the lower link follows a target trajectory. Further, the controller executes a first abnormality management process in which the controller shuts off torque transmission from the actuator to the user when the controller detects an abnormality before outputting the commands to the actuator.