Three or more displacement generation mechanisms, which allow some of a plurality of guide members that are provided respectively to a plurality of guide holding members that is capable of reciprocally moving substantially perpendicularly to shifting directions of a reciprocally movable linear motion structural member with respect to a base member, and a rocking member that is provided to an end of a linear motion member capable of reciprocally moving in directions substantially in parallel with the shifting directions of the guide holding members so as to rock around an axis substantially perpendicular to the respective shifting directions of the linear motion structural member and the linear motion member, to be made in contact with each other so that some of the guide holding members are displaced relatively to the linear motion structural member, are operation-controlled, with a result that a transmission gear ratio between a displacement velocity of the linear motion member and a displacement velocity of the linear motion structural member is changed.