A pneumatic multi-chamber rubber tube actuator was designed, fabricated, and tested. Applying pneumatic pressure to each chamber sequentially causes periodical motions of the rubber tube surface tracing a polygon shape. By winding a pair of the tube actuators in parallel spirally around a colonoscope, they generate deformation waves around the colonoscope traveling along its axis, cause the locomotion force, and work to assist colonoscope insertion to large intestine. The basic idea and structure of the actuator and the experimental results showing their potential as a linear actuator assisting colonoscope insertion were reported in the previous paper. However, improvements of actuator designs in detail are still necessary to increase their locomotion ability. In this report, after surveys and evaluation of several types of this rubber tube designs, optimizing designs were made. The evaluation and designs are made based on non-linear FEM analysis. We successfully obtained an optimized actuator design which generates the motion 15.5 times larger than that of the actuator reported in previous report. The prototype shows good performances.