The applications of pneumatic artificial muscles (PARM) are increasing in the power assist and manipulator systems, because of its advantages such as high power-to-weight ratio and flexibility. Usually, we have to measure the contract ratio of PARM using a displacement sensor installed near the actuator which is not easy to make dust and water proof. In this paper, we propose an estimation method of the contract ratio from the volume change of the PARM which can make the actuator part sensor-less. The air charged and discharge to the PARM is measured by a quick response laminar flow meter. Then, the volume of the PARM is estimated from the flow rate and the pressure measured by a pressure sensor. In order to estimate the contract ratio accurately, thin copper wires are stuffed to the PARM to produce an almost isothermal condition during the charge and discharge. The effectiveness of the proposed method was confirmed experimentally.