A rotating knob is rotatably attached to an outer peripheral portion of an operation lever. The rotating knob has a lead thread portion on the inner surface thereof, and the lead thread portion meshes with external teeth of a ring gear. Internal teeth of the ring gear mesh with speed-increasing gears, and permanent magnets are fixed to the speed-increasing gears. A circuit board is arranged in the operation lever so as to extend along the axial direction thereof, and magnetic sensors mounted on the circuit board are opposed to the permanent magnets at positions close thereto. Accordingly, the operation position of the rotating knob can be detected by the magnetic sensors. The gears, the circuit board, etc., are supported by a support case and a cover case to form a switch unit.