Abstract: A swash-plate hydraulic pressure device for use as a hydraulic pump or motor includes a cylinder block having an annular array of cylinder holes defined therein around an axis. A plurality of plungers of a ceramic material are reciprocally movably disposed in the cylinder holes, respectively, the plungers having respective partly spherical tip ends. A swash plate of metal is disposed around the cylinder block for rotation with respect to the cylinder block, the swash plate having a annular groove of a partly spherical cross section defined therein and, the partly spherical tip ends of the plungers engaging in the groove. The tip ends of the plungers preferably have a pore area percentage of at most 7.8% and a maximum surface roughness of at most 1.6 s. The partly spherical tip end of each of the plungers has a radius R.sub.0 of curvature, each of the plungers has a diameter D, the groove has a radius R.sub.1 of curvature and a depth E. The ratios R.sub.0 /D, R.sub.0 /R.sub.1, and E/R.sub.