An external axial rotary piston blower having a housing including a casing part and two side parts of which the casing part encompasses a cylindrical runway surfacing which is formed by two cylinders intersecting each other in a region of an inlet and an outlet and in which two identical pistons having at least one wing portion rotate among each other, such pistons having starting surfaces with a large radius running against a casing runway surfacing and respectively another piston having a starting surface with a small radius running correspondingly against a casing runway surfacing. Shafts of the pistons are offset or shifted in cold condition in a direction of a short housing axis toward the outlet and the housing on the side of the outlet is shorter in axial direction in cold condition than on the side of the inlet.