This present invention relates to a preparation of gaseous fuel (natural gas for example) for its further transfer under pressure to fuel tank of a vehicle 22. This object is achieved by a method for compressing gas by alternate transfer of gas into two vertically arranged compressing vessels 1 and 2, its compression and forcing into high-pressure vessels by filling the compressing vessels 1 and 2 with working fluid 30 under pressure by means of a hydraulic drive 5. A novelty of this method lies in that, each cycle of gas 29 compressing and its forcing out of the compressing vessels 1 and 2 is performed until these vessels are fully filled with the working fluid 30 contained in the compressing vessels 1 and 2 and alternately forced out of one compressing vessel into the other in response to a signal sent by fluid-level sensor 4.