Pneumatic artificial muscles have advantages of lightweight, strong force, and electrical power saving for applications to power-assist systems or care support systems. These applications require precise control of artificial muscles. The artificial muscles have non-linear characteristics because they are mainly composed of elastic materials. The characteristics make the precise control difficult. In this paper, we propose a stretchable strain sensor for an application to pneumatic artificial muscles. This strain sensor has the characteristics of stretchability, length measurement, and lightweight, and can directly measure the contraction of the artificial muscle by attaching the sensor on the muscle. We describe the structure of the sensor and the principle, and show the fundamental characteristics. In addition, we confirm the next two characteristics. The sensor stops the contraction of the muscle at an error of 2.1 mm, and has no hysteresis in a loop of contraction and stretch.