Many kinds of pneumatic cylinders have built-in cushions to reduce impact force caused by the kinetic energy of load mass at the stroke end. The driving speed of pneumatic cylinders has become so faster than before, and it is necessary that the capacity of the built-in cushion is expanded. The conventional built-in cushion uses only adjustable bleed orifice, on the other hand a new type built-in cushion, which has been developed as high performance one, uses a relief valve. It enables to absorb impact force much better than conventional one. However, details about the built-in cushion have been unclear. We have examined basic characteristics of the built-in cushion with relief valve and factors which influence its performance, and considered guidelines for its design. Then we have investigated an improvement of its mechanism. The paper reports these results, which have been acquired according to experiments and computer simulations.