Flexible and stretchable electronics are expected to contribute to electric conductors, sensors and actuators for various applications, such as robotic systems and power-assisted systems. In this study, a stretchable strain sensor is proposed as a length sensor for a pneumatic artificial muscle. To measure the length of the artificial muscle, the stretchable strain sensor is required to have quick response characteristics. A capacitance measurement circuit based on a band-pass filter is analyzed, and is tuned by the parameters of its electric components. The circuit measures the capacitance of the strain sensor with accuracy equivalent to a commercially available LCR meter. We confirm that the combination of the capacitance meter and the strain sensor measures the length of the artificial muscle with a high sampling rate of 10 kHz.