To absorb a large shock and effectively utilize a space by absorbing the shock when the stroke of a piston in an air cylinder unit is completed with the outer tube of an oil damper kept in close contact with a cylinder outer periphery section at the front and rear end sections and moved in the axial direction by external force.

When the outer tube 26 of an oil damper 3 kept in close contact with a cylinder outer periphery section at the front and rear end sections and moved in the axial direction by external force is lowered in the direction of an arrow 1, oil 35 passes through an orifice hole 23 formed on a fixed piston 25 from an upper oil chamber S2 surrounded by an inner sleeve 21, an outer sleeve 31, a rear end guide 35, and the fixed piston 25, then it is moved to a lower oil chamber S1 surrounded by the inner sleeve 21, the outer sleeve 31, a front end guide 32, and the fixed piston 25. Damping force is generated by the resistance existing when the oil 35 passes through the orifice hole 23, and the shock applied to the outer tube 26 is absorbed.