In a pneumatic compressor for the compression of waste, for example waste paper comprising a cylinder (1) with a piston (2), which via a piston rod (4) activates a compression plate (3), which is slidably mounted in a compression chamber (5) which has a top door (6) through which waste paper is thrown into the compressor, and a bottom door (7) through which a finished bale can be taken out, which compression takes place in a series of compression cycles, which each time is charged with a new portion of waste, until the waste can offer a counter pressure corresponding to the pressure which the piston (2) can exert at a pressure of 8 bars. In 80 - 90% of the compression cycles necessary for the compression of a bale, the piston (2) reaches its bottom position in the cylinder, and each time the pressure reaches a pressure of 8 bars in the cylinder (1). According to the invention a partial control system is incorporated a control system which comprises a closing valve (18), which closed for admission of compressed air to the cylinder (1), when the piston reaches it bottom position. Consequently, the pressure developed in the cylinder (1) will in 80-90% of the compression cycles be reduced considerably. There will thus be a corresponding reduction of the energy consumption and the noise level.