PURPOSE: To miniaturize the above hydraulic cylinder by disposing a pressure intensifying piston whose part is inserted in a heat side hydraulic pressure chamber of a main cylinder from a main cylinder outer peripheral part directly between a main piston and a main piston rod projecting side edge plate of the main piston.

CONSTITUTION: A pressure intensifying tube 5 formed integratedly with a pressure intensifying piston 4 is accommodate in a main cylinder 1 slidably, a main piston rod 3 is made to pass through a central hole of the pressure intensifying piston 4 slidably, and a main piston 2 is accommodated in the pressure intensifying tube 5 slidably. Besides, an oil chamber 12 on the main cylinder 1 bottom side of the main piston 2 is communicated with the third port 13, an oil chamber 8 on the pressure intensifying piston 4 side is communicated with the second port 9 through a through hole 10 and a groove 11 of the pressure intensifying tube 5, and a pressure intensifying oil chamber 6 between the pressure intensifying piston 4 and a rod side edge plate 1a of the main cylinder is communicated with the first port 7. It is thus possible to miniaturize only by adding the pressure intensifying pistons 4, 5 inside the main cylinder 1.