A retention mechanism according to an embodiment includes a fixed part including a magnet, and a movable part opposed to the fixed part. A plurality of rolling elements are provided between the fixed part and the movable part, and the movable part is movable in a plane direction. A magnetic material which forms, in combination with the magnet, a magnetic spring is provided on the movable part. The magnetic spring generates a magnetic attraction force which attracts the movable part to the fixed part. The magnetic material is extended along a direction in which magnetic poles of the magnet are arrayed.