Thin artificial muscle is a flexible and lightweight pneumatic actuator with 2 mm in outer diameter. It generates contractile displacement by applying pneumatic pressure. We have fabricated a string-shaped actuator called “Active string” by accumulating thin artificial muscles using the string production process. The active string is expected to be applied to mechanical systems used in medical, welfare, and agricultural fields because of its high contractile displacement/force, flexibility, and safety. However, displacement control of the active string is challenging because general bulky and rigid displacement sensors such as an encoder and a potentiometer are not suitable for the sensor element of the active string. These sensors are difficult to embed into the active string, and their rigidity interfere the advantage of the active string. Therefore, in this report, a flexible optical fiber sensor is combined with the active string to enable real-time sensing of its displacement. The optical fiber is easy to embed to the active string and does not interfere with the motion of the active string. As the active string contracts, the radius of curvature of the optical fiber decreases, and light intensity propagating in the optical fiber decreases due to bending loss. Fundamental experiments were carried out. The experimental results showed that the sensor value acquired by the embedded optical fiber sensor changed with corresponding to the displacement of the active string. It suggests that it is possible to estimate the displacement of the active string by the optical fiber sensor.