To provide a system for floating an object such as a conventional airplane, automobile or robot in the air with buoyancy generated by operating a hydraulic device with power of the object.

A hydraulic cylinder is operated by using a spring or a telescopic body for a connection body for connecting the total weight of the object to a member integrated with or fastened to a driving part of the hydraulic cylinder 5 to be driven, to generate the buoyancy so as to allow vertical taking off and landing. After the object once floats in the air, the hydraulic device 1 is operated by utilizing recovery energy of a head wind generated with propulsive force of a propeller 6 or the like to continuously generate the buoyancy. Thus, energy is drastically saved, and safety is ensured.