Shock absorber having self pumping unit

A shock absorber, comprising:

• a piston rod;
• a piston valve coupled to one end of the piston rod;
• an inner shell having an inner space configured to receive the piston valve, the piston valve being operable to reciprocate between rebound and compression strokes, the inner space of the inner shell being divided into a rebound chamber and a compression chamber by the piston valve;
• an outer shell positioned outside of the inner shell and having a reservoir chamber, the reservoir chamber being configured to supply oil to the compression chamber in the rebound stroke and to recover oil in the compression stroke;
• a high pressure tank having a high pressure chamber installed outside of the outer shell;
• a self pumping unit for sucking gas external to said self pumping unit and for pumping the sucked gas into the high pressure chamber using, via a mechanical connection to the piston rod, energy generated by reciprocating movement of the piston rod;
• and
• a housing provided on a lower portion of the high pressure tank to slidably overlap with an outer circumference surface of the high pressure tank, the housing having at least one expandable tube selectively communicating with the high pressure chamber, the high pressure chamber being provided on an outer circumference surface of the outer shell.