According to one embodiment, a holding mechanism includes a first holding part, a second holding part, a first guide, a second guide, a third guide, a fourth guide, and a driving mechanism. The second holding part faces the first holding part in a first direction. The first guide is connected to the first holding part and capable of moving the first holding part in the first direction. The second guide is connected to the second holding part and capable of moving the second holding part in the first direction. The third guide is capable of moving the first guide in the first direction. The fourth guide is capable of moving the second guide in the first direction, and aligned with the third guide in the first direction. The drive mechanism changes a distance between the first holding part and the second holding part.