The aim of this study is to evaluate the breast stroke motion using new measurement system and analysis method. The measurement system consists of 3-axis gyro sensors, 3-axis acceleration sensors, and 3-axis magnetic field sensors and it is attached to the subject's body. The analysis method corrects the drift error of gyro sensor and provides joint angle using sensor fusion and inverse kinematics. The information about swimmer's motion in hydrospace is easily obtained by using this system. The experimental results indicated the breast stroke motion quantitatively. This measurement system and analysis method can analyze the major features of swimming motion in breast stroke. This method may be able to quantitatively evaluate the skill of swimmer, and to apply to other swimming strokes.