the inner peripheral part 15 of the resilient seal ring r whereby an excellently high sealability is ensured with the resilient seal ring r firmly held around the annular piston ring groove 4 the piston ring groove 4 is additionally formed with an annular groove 17 at its other side lower side as seen in fig3 said annular groove 17 being located opposite to the rear side of the inner peripheral part 15 of the resilient seal ring r and having a depth more than that of the piston ring r so that the innermost peripheral edge of the inner peripheral part 15 of the resilient seal ring r in area c in fig3 is engaged to the annular groove 17 as a result the embracing section h of the resilient seal ring r has an increased embracing force exerted on the bottom face 4a of the annular piston ring groove 4 and at the same time inhibits idle displacement of the seal ring r in the vertical direction as seen in fig3 next operation of the sealing device in accordance with the illustrated embodiment of the present invention will be described below as the piston 3 effects downward movement from the position as illustrated in fig1 the delivery valve 11 is caused to close and thereby pressure in the pumping chamber a is reduced.