A leg assist device with a control law suitable for assisting standing up motion is provided. The leg assist device is provided with an upper leg link, a lower leg link, a rotary joint, and a controller. The upper leg link is attached to an upper leg of a user and the lower leg link is attached to a lower leg of the user. The rotary joint rotatably connects the lower leg link to the upper leg link. The rotary joint also has an actuator which rotates the lower leg link. The controller controls the actuator so that the lower leg link angle matches a target angle. The controller has a torque limiter that limits the magnitude of the command torque that is output to the actuator. The controller sets a standing position angle corresponding to a users standing position to the target angle, and raises an upper limit of the torque limiter as a users hip height rises.
