A sealing cup to be used for a hydraulic piston-cylinder device the cup having an outer lip with a leading edge adapted to slide along a wall when fitted in the device. The angle .alpha. of the leading edge with respect to the cylinder axis is made smaller than the angle of a portion other than the leading edge with respect thereto, and is made as 1.degree..ltoreq..alpha..ltoreq.15.degree. in the free condition of the cup. When the sealing cup is installed to be slid in the cylinder, a wedge-shaped clearance is formed between the leading edge and the cylinder wall due to provision of the angle .alpha., thereby admitting the fluid therein to decrease the slide resistance of the sealing cup.