In this study, we have been researched a very lightweight soft actuator with a folding structure made of plastic films. This actuator has several unique characteristics which are very lightweight, flexible, low-pressure drive. Some applications such as the wearable robotic and life support robots are expected. However, the force characteristics and the design method of the force don't make a clear. Because, the dynamic model of this actuator is complicates by wrinkles of pleated structures. In this paper, we focused on both the folding amount of a pleat part and the actuator size. We defined the pleat part to a rotational spring of variable stiffness. It is an object to establish a design method by deriving a static force characteristic experimentally.