In a cup seal of the present invention, an inner lip (27b) is pressed into close contact with a piston (9) and an outer lip (27c) is pressed into close contact with a bottom wall (26a) of a concavity (26) by fluid pressure developed in a fluid pressure chamber (11) when the piston (9) moves forward. Therefore, fluid pressure is sealed. When the piston (9) is retracted, the outer lip (27c) is deflected inwardly and is thus spaced apart from the bottom wall (26a) of the concavity (26). Hydraulic fluid of a reservoir is supplied to the fluid pressure chamber (11) through a first communication path (22), grooves (27e), and the space between the outer lip (27c) and the bottom wall (26a). As mentioned above, the inner lip (27b) has only the sealing function, while the outer lip (27c) has both the sealing function and the pumping function and ensures sufficient fluid supply by the grooves.