A free-piston compressor which has a cylinder, a compressor piston disposed movably in the cylinder, thereby first and second chambers being formed in the cylinder, a low pressure pipe coupled to the first chamber through which pipe, low pressure fluid flows, a high pressure pipe coupled to the first chamber through which, high pressure fluid flows, two valves coupled between the low pressure and high pressure pipes, and the second chamber, a control device for controlling the two valves in a manner that a dead point position of the compressor piston is kept at a target point, and driving means for driving the compressor piston.