A compressor includes pistons slidably accommodated in cylinder bores. A head and a skirt are formed at the ends of the piston. The head has a peripheral surface that always contacts the inner surface of the cylinder bore. The skirt is coupled with a swash plate. An annular space opens to the side of the piston. The annular space is located between the head and the skirt. A recess is located on the inner wall of a front housing and extends longitudinally in the axial direction of the piston. A projection is integrally formed on the skirt of the piston to engage with the recess. Rotation of the piston about its axis with respect to the cylinder bore is restricted by the engagement of the projection with the recess. A force acting on the piston in a direction transverse to the axis of the piston is received by the recess through the projection.