PURPOSE: To enable mass production at a low price and the high accuracy less than micron by forming a rod member, which one end elastically abuts to a cam plate side surface, to perform a linear motion with the other end thereof in the axial direction with the rotation of the cam plate formed with an involute curve.

CONSTITUTION: A rod member 6 which shaft core 12 coincides with a contact line 13 of one point on a circumference in a base 10 is supported so that one end thereof always and elastically abuts to profile on a normal of an involute curve forming a cam plate in a side surface of a cam plate 5. Some parallel supporting springs 14 such as springs in spiral or spider-feet-shape are located in the predetermined places of the rod member 6 to support the rod member 6 reciprocatably. Further, a member having a small friction coefficient such as a ball 15 is fitted to the cam plate 5 abutting part of the rod member 6. The cam plate 5 is formed with a certain involute curve 11 like this, and the rod member 6 is supported by the cam plate 5 reciprocatably in a certain relation. Consequently, the predetermined effect that a moving length of the rod member 6 abutting to the cam plate 5 is in proportion to a rotary angle of the cam plate 5 is displayed.