To provide a swing preventive device for a sliding door capable of preventing a swing in the longitudinal direction of the sliding door and capable of easily adjusting a position in the longitudinal direction.

In this swing preventive device 1 for a sliding door, a position adjusting mechanism 20 for horizontally moving a guide member inserted into a groove formed in an upper part or a bottom part of a sliding door 5 is arranged on the sill T or a lintel, the position adjusting mechanism 20 rotatably constitutes a disk-shaped member for supporting the guide member, a gear is formed on the periphery of the disk-shaped member, an adjusting gear for meshing with this gear is arranged, and the disk-shaped member is rotated by operation of the adjusting gear to change a position of the guide member 30.