PURPOSE: To lightly perform lift-driving action of a jack with a relatively simplified equipment by providing constitution such that compressed air, branching from a compressed air pipe existing in a trailer, is used as energy for lift-driving the trailer jack.

CONSTITUTION: In a trailer jack, an auxiliary leg 8, lifted through a gear box 11 by rotating a manually operated handle shaft 14, is provided in a trailer main unit 2. Here mounting an air motor 15 to the gear box 11, a gear 17, mounted to an air motor shaft 16 of the air motor 15, is meshed with gears 14a, 14b on the manually-operated handle shaft 14, selectable in a low or high speed transmitting route, in the gear box 11. The air motor 15 is connected to a branch pipe 22, which branches from a compressed air pipe 21 existing in the trailer main unit 2, through a compressed air introducing part 20 and a selector valve 19. By selectively controlling this selector valve 19, the air motor 15 is normally or reversely rotated so as to lift the auxiliary leg 8.