To provide shake preventing structure of a sliding door capable of preventing or reducing the occurrence of unpleasant sound irrespective of the level difference shape of the sliding door even if there is a level difference at a lower rail of the sliding door.

An auxiliary rotating wheel 12 including a main rotating wheel 11 is journaled to a post shaft 12 of a shake preventing mechanism 10, and the auxiliary rotating wheel 12 is formed with an open section 33 opening at least a position where the main rotating wheel 11 faces the lower part of the sliding door 1. Cushioning surfaces 34, 35 facing the lower part of the sliding door 1 and capable of contacting a step part 1a are formed at both circumferential ends of the open section 33, and the radius of curvature of the cushioning surfaces 34, 35 is formed larger than the radius of gyration of the main rotating wheel 11.