Abstract: An auxiliary device of a movable plank, wherein the movable plank is moved between a first position and a second position, includes a sliding member, a driving member and a power providing device. The sliding member is provided adjacent to the frame and incident to a screw pair. The driving member is provided adjacent to the sliding and incident to a screw pair. The power providing device is provided adjacent to the sliding member. The power providing device stores power when the movable plank is moved to the first position, and releases the power to move the sliding member when the movable plank is moved to the second position from the first position.