PURPOSE: To provide a hydro-pneumatic pressure transformer of a structure which does not cause an apparatus to have in internal leakage of the working fluid impairing the apparatus operations.

CONSTITUTION: A pressure accumulator piston 11 is provided between a working fluid and a compressed air, and a plunger 21 penetrates the center of the piston 11. For separating oil from air, ring-shaped grooves 38 and 42 are provided at the outer surface of the piston 11 and in the wall of the center hole 41, in an arrangement via which is in communication with each other and also to the outside via an exhaust hole 39 leading to the outside.