Mode controller

A mode controller in which a step-index optical fiber (2a) and a graded-index optical fiber (2b) are connected and an optical fiber connection body (2) is formed, the optical fiber connection body (2) is wound on a plurality of bobbins (3a, 3b, 3c, 3d) in one or more sets of two thereof to form the mode controller, and the optical fibers are twisted to form helical parts (2a1, 2b1), light being incident on the step-index optical fiber (2a) and a propagation mode being converted to a balanced mode distribution, and then light being incident on the graded-index optical fiber (2b) and the mode being converted to an arbitrary lower order mode, wherein the mode controller is capable of emitting NFP light exhibiting EF conforming to the EF boundary condition described in the IEC 61300-1 international standard.