PURPOSE: To reduce a separating response time in an attractive plate to a momentary length so as to enable high speed action of a positioning device to be realized, by forming pressure reducing pockets, which enables reduced pressure attracting force to urge the attractive plate, in a back surface of the attractive plate.

CONSTITUTION: A lock mechanism main unit 2, providing a supporting wall 23 through fine adjusting springs 22 for a mounting base plate 21 fixed to a movable part 1, mounts an attractive plate 4 between opposed supporting pieces 24 of the supporting wall 23 through lock springs 3. The attractive plate 4, being formed in a flat plate shape in thickness a little smaller than a distance between a fixed base 5 forming an attracted surface and the supporting wall 23, forms a predetermined gap (g) between the plate 4 and the base 5 or between the plate 4 and the supporting wall 23. The attractive plate 4 forming pressure reducing pockets 41, 45 of fixed depth respectively on surfaces opposed to the base 5 and the supporting wall 23, is normally urged by the lock springs 3, 3 to a side of the supporting wall 23, holding the gap (g) to be formed between the base 5 and an opening surface of the pocket 41.