To provide a device which is simple in structure and easy in a changeover between a fixed and a variable state at a relative position of a plurality of relatively moved members.

A brake motion member 50 has a pair of pressure parts 62, 62 severally having a pressure face 60 arranged opposite to a side 58 of a brake bar 54, and the pressure part 62 is connected to a connection part 66 through a plate spring part 64 to make each pressure face 60 elastically deformable toward the side 58. In addition, an inclined outer wall face 68 of the pressure part 62 is brought into slide contact with a pair of inclined inner wall faces 56 of a housing 48. When the brake motion member 50 is moved upward according to an energizing force of a coil spring 72, the pressure part 62 is pressed by the inclined inner wall face 56 to turn into a braked state where the brake- bar 54 is held between them. On the contrary, when a pressure pin presses the pressure part 62 to move the brake motion member 50 downward, a released state where the brake bar is not sanswiched by the pressure part 62 is formed.