The authors developed a wearable finger motion measurement system using inertial and geomagnetic sensors. Using this system, motion and posture of the hands and fingers can be measured. However, the joint center and segment axis cannot be accurately measured in a previous study using the sensors. Therefore, the authors proposed a method of estimating the joint center and segment lengths. This method utilizes the fact that the calculation formula of the rotational acceleration in the sensor's coordinate system is the vector product. However, because the vector product is irreversible, the rotation center was calculated by using a position vector placed on an intersection line of two planes constituted by rotational acceleration. As a result of the verification, estimation error was small. In addition, finger motion was measured using posture measurement and rotation center estimation. This measured motion, joint angle and segment lengths estimated by the finger motion measurement system were compared with a motion capture system. As a result, the initial MP and PIP joint angles had some differences, but there was no difference for the MP and PIP joint angles during movement and the other joint angles. Therefore, we need to improve the estimation method of the PIP and DIP joint centers.