For safety contact between humans and machines, Soft actuators are very useful. This reason is that soft actuators have flexibility, light weight and safety. However, it is difficult to realize accuracy control. The aim of this research is to develop a pneumatic soft actuator that has a flexible displacement sensor. The soft actuator is a pneumatic artificial muscle that can generate contracting motion. It consists of a silicone rubber tube and PET plates. Additionally it has a displacement sensor on constructional material. In this research, we have verified two types of sensors, strain gages and conductive film sensors. The both sensors can detect the change of their electrical resistance when the PET plate in the actuator is deformed. Through the experiments we have compared with each sensor's feature.