A shock absorber is attached coaxially with a reciprocating rod driven by a reciprocating unit to prevent a bending moment from being applied to the reciprocating rod in absorbing an impact force. The shock absorber has a hollow rod and an outer cylindrical body mounted axially movably relatively to and outside the hollow rod. An accommodating space is formed between the hollow rod and the outer cylindrical body. The hollow rod carries an annular piston, which partitions the accommodating space into two liquid chambers. A compression spring biases the outer cylindrical body toward one end of the hollow rod. When an impact forces the outer cylindrical body toward the other end of the hollow rod, liquid flows from one of the liquid chambers to the other through a gap, so that a resistance force is applied to the annular piston.