An apparatus for manufacturing a three-dimensional object by a layer-by-layer solidification of building material at the points corresponding to the cross-section of the object to be manufactured in a respective layer. The apparatus includes a process chamber in which the object is to be built up layer by layer by selectively solidifying layers of a building material in a build area, a gas supply device, and a recirculating air filter device, wherein the apparatus comprises a pressure stabilization device configured to keep the pressure in the process chamber substantially constant.