PURPOSE: To make balance keepable automatically by detecting a tilt angle of load supported on a hydraulic jack, and outputting a compensating signal in direction of making the detected tilt angle smaller, to a solenoid proportional valve.

CONSTITUTION: The maximum permissible value of tilt angles in all directions, which makes an airplane 1 regardable as horizontality in an automatic operation mode is preset in a controller 19. The tilt angles in all directions are detected by an inclinometer 20 installed in the airplane 1. When the signal is inputted, the tilting direction is discriminated from the detected value, and using a hydraulic jack 5 as a master jack, such a compensating signal as adjusting hydraulic fluid rate in direction of making the tilt smaller is outputted to a solenoid proportional valve 13 of one side or both sides of other hydraulic jacks 6, 7. Output of this compensating signal is stopped when the detected value of the inclinometer 20 is restored to the setting range. Consequently, the airplane 1 is automatically kept in horizontality and moved up and down.