A prestressing apparatus for tensioning reinforcement cables in recesses or shafts of structures has a main cylinder-piston unit (15, 17) and a counterforce cylinder-piston unit (16, 22). The two pistons (17, 22) are connected via their piston rods (39, 40) to a fixed arm (2), and the cylinders (15, 16) are connected to a movable arm (10). The main cylinder-piston unit disposed in the middle of the arms and the off-centered counterforce cylinder-piston unit are double-acting. The piston chambers (33, 35, 36, 37) are hydraulically cross-connected. Each side of the counterforce piston has half the piston area of the corresponding communicating side of the main piston, so that the prestressing force (V) and the counterforce (G) are each one-half the pressing force (P). The free ends of the arms extending beyond the main cylinder-piston unit serve to transmit force to the reinforcement