To maintain the fit condition of a slipper with a plunger body when a cylinder is in high speed rotation while the oil in a cylinder hole is in a low pressure.

A plunger assembly comprises a slipper 205 of cup form having a convex spherical outside surface and a concave spherical inside surface, a plunger body 201 having a spherical recess 201b fitted slidably on the convex outside surface, a head 212 in slide contact with the concave inside surface, a rod member 211 extending being tied with the head 212, and a coupling member 210 coupled with the plunger body in penetrating the slipper. Inside the plunger body 201 a pressing force giving mechanism 220 is installed to transform the centrifugal force acting with rotation of a cylinder into an axially directed force, give the resultant to the coupling member 211, and press the plunger body 201 to the slipper 205.