A piston power transmission apparatus is disclosed having an upper cylinder block, a lower cylinder block, a vertical wall mounted in the upper cylinder block to provide an internal chamber and external chamber respectively, the top piston being slidably housed in the internal chamber and a ring-type piston being slidably fitted in the external chamber, and a bottom piston being slidably mounted in the lower cylinder block with a diameter larger than that of the top piston, a compressed air tank which is communicated through a guide pipe with an external chamber causing the ring-type piston to move downward as soon as the pressure in the internal and external chambers falls below the air tank pressure, wherein the bottom piston and the top piston are connected to each other by the piston rod guide, and the space enclosed by the top piston, the ring-type piston and the bottom piston is filled with an incompressible fluid oil.