In a rocking piston type compressor, a piston rod 11 is rockingly slidable within a cylinder 10. The compressor is provided with a lip ring 20 and an expansion member 30. The lip ring 20 is configured to seal between the cylinder 10 and piston rod 11. The lip ring 20 is provided on a leading end portion 13 of the piston rod 11 and includes a bottom portion 21 in a circular plate shape and a lip portion 22 extending from an entire peripheral edge of the bottom portion 21 to an upper side in a height direction. The expansion member 30 is provided on an inside of the lip portion 22 and configured to outwardly expand the lip portion 22 at a side of an upper portion in the height direction of the lip portion.