Controlled by a microcomputer, a pneumatic system in which several cylinders are connected with each other in series, can realize accurate positioning. When the strokes of the cylinders in some unit vary in a sequence of 211-1 where n is an integer number beginning from 1, the positioning can be realized step by step in this unit from 0 to 2N-1 where N is the number of the cylinders used in the system. Such systems have no feedback control and no electric device around the actuator, so they are usable in various working conditions even the conditions are so hazardous such as in explosive gas or in a strong magnetic field.This research was carried out experimentally by means of a model system in which four cylinders were used. From the research, the effects of elastic external load on the system have been made clear.