Use of the flat type strain wave gearing increases in industrial robots owing to its shortness and its high torsional stiffness. However, the vibration characteristic of the flat type strain wave gearing has not yet been adequately clarified. In this paper, the robot system, which was composed of an input motor, the flat type strain wave gearing and an output arm, was operated to clarify the effects of composite errors in meshing between the flexible spline and two circular splines and the misalignment of two circular splines to the flexible spline on the amplitude of vibrating displacement of the output arm.