Abstract: A semi-spherical shoe 1 has a convex semi-spherical surface 1A which is disposed in sliding contact with a semi-spherical recess 2A formed in a piston 2. The convex semi-spherical surface 1A is defined by a convex surface of a revolving body which is formed when an arc of a circle having a radius of R2 and having a center C offset from the axis L of the semispherical shoe in an direction orthogonal thereto is rotated about the axis L as a center of rotation. A lead-in 1G which is arcuate in section is formed toward the top of the convex semi-spherical surface 1A and maintains a clearance 7 with respect to the semi-spherical recess 2A. A lubricant oil which is stored in a space 4 is smoothly supplied onto a sliding surface 1F through the clearance 7. The invention provides a semi-spherical shoe 1 having a good sliding response and an increased length of useful life.