A shock absorber is attached coaxially with a reciprocating rod driven by a reciprocating unit for absorbing an impact force without bending the reciprocating rod. The shock absorber has a hollow rod provided with a through hole. The hollow rod is mounted within an outer cylindrical body. An accommodating space is formed between the hollow rod and the outer cylindrical body. A spring force toward one end portion side of the hollow rod is applied to the outer cylindrical body by a compression coil spring. An annular piston provided in the hollow rod partitions the accommodating space into two liquid chambers. When the impact force is applied to the hollow rod, the liquid flows from one liquid chamber to another liquid chamber through a gap, whereby a resistance force is applied to the hollow rod.