We are developing a XENMA pelvis rehabilitation robot for the gait motion which can generate periodic motion with specific variable crank structures. We are focused on trigonometric property of pelvis motion on gait. We designed robot for patient to ride it, and control his pelvis with two crank-slider mechanisms and variable crank structure that can control its crank length. We also designed modified inverse kinematics methods for a variable crank structures with both geometrical and resolved motion rate control solutions. Modified IK method is simulated and result shows our solution is valid and makes better following performance.