PURPOSE: To keep the attitude of an airplane such as a helicopter horizontal regardless of the inclination of the ground by providing landing gears with the right and left leg support sections expanded or shrunk by the oil pressure, and controlling the oil pressure in response to the inclination of the ground.

CONSTITUTION: When a helicopter is to land, electrical signals such as the attitude of an airplane detected by a sensor 5, the lengths of leg support sections 3 detected by potentiometers 21, and pressures of the oil 17 in cylinders 15 detected by pressure gauges 22 are inputted to a computer 23, and control signals, which change the lengths of the right and left leg support sections 3 in response to the attitude of the helicopter so that the helicopter holds the horizontal attitude, are generated to control valves 25, 31. When the leg support section 3 is to be extended, the valve 31 is closed, the valve 25 is opened, and the high-pressure oil in an oil reservoir 24 is fed into the cylinder 15. When the leg support section 3 is to be shortened, the valve 25 is closed, the valve 31 is opened, and the oil 17 in the cylinder 15 is guided into an oil reservoir 32.