To assemble a key lock collar having the diameter larger than the inside diameter of a sensor coil, and to easily assemble a worm wheel without increasing the number of components.

A small diameter sensor coil 13 is passed between steering shafts (an input shaft 3 and an output shaft 6), and a worm wheel 15 having the diameter larger than that of the coil 13 and a key lock collar 4 having the diameter larger than that of the coil 13 are mounted on both end sides. In this situation, the worm wheel 15 is assembled later to the steering shafts (the input shaft 3 and the output shaft 6) passing through the sensor coil 13 with the key lock collar 4 fixed thereto.