In this study, we describe a manipulator that induces a horizontal surface rotation motion of the shoulder so as to change the phase difference between the shoulder and the upper arm. We constructed an algorithm that estimates the time of heel contact while participants walk on a treadmill, and controls the axial rotation angle of the shoulder at that time. We verified the accuracy of the proposed algorithm via experiment with healthy young participants, and found that it could control the angle of the manipulator. The experimental results showed that the amplitudes of the upper limbs' center of mass trajectory were increased significantly when the phase difference between the shoulder and upper arm was between 0 [°] to 20 [°]. This study shows that it is possible to train people to change the phase difference between their shoulder and upper arm, and thereby increase the amplitude of the upper limb swing. In the future, we will verify this experiment in the older population.