A numerical analysis of a piston motion in a free piston driver has been conducted including a diaphragm rupture. Quasi-one-dimensional flow is assumed in the driver. Various losses, which are generated in a compression process, are simplified. A process of the diaphragm rupture is assumed by Outa’s model. Experiments are also conducted by using a free piston shock tunnel. Calculated pressure histories show excellent agreements with experimental results. The numerical analysis shows that the piston motion is strongly affected by the diaphragm rupture. Especially, pressure histories in front of the piston change significantly with compact compression tubes. Diaphragm opening time is necessary to be considered to increase test time and to obtain soft landing conditions in compact free piston drivers.