Soft pneumatic pouch actuators with simple structures and fabrication processes are being studied for application in soft robots. In addition, some research groups have tried to add sensors to the pouch actuators to estimate their states. In this study, we propose to embed a soft strain sensor made of liquid metal in the bladder of a pouch actuator made of stretchable styrene–ethylene–butadiene–styrene (SEBS) film, which can expand to a large extent under pressure to enable shape estimation. A method of processing the bladder of the pouch actuator to convert it into a sensor has yet to be proposed. We developed a fabrication process to form the small strain sensor in the SEBS film and evaluated the strain sensor’s stretch characteristics. Finally, we confirmed that the deformation of a bending pouch actuator caused by pneumatic pressure is expressed in the electrical resistance of the embedded strain sensor.