A compressor which includes a piston (150) reciprocating in a cylinder bore (111) provided in a cylinder block. Length of the circumferential surface at the compression load side (160) is made to be longer than that at the anti-compression load side (170), so that area of sliding-contact surface at the compression load side is greater than that at the anti-compression load side. The above configuration is effective to prevent occurrence of wearing due to unsymmetrical contact of piston with cylinder bore. Thus, deterioration in the refrigeration capability and instability of the performance is prevented.