To provide a belt displacement preventing device for automatically restoring a displaced belt into an original center position so that the belt normally travels at all times.

When a timing belt 12 is displaced and a roller 22a is pushed, a first support 24a is rotated around a shaft 25a clockwise viewed from the lower side. A second support 30a is rotated around a shaft 32a through a connector 28a counterclockwise viewed from the side face. Then, a spherical bearing 33a supporting a shaft 34 of a driven pulley 17a is moved downward and the driven pulley 17a is inclined. The timing belt on the displaced side is pulled, and reaction from the driven pulley 17a works on the timing belt 12 from the displaced side to the inside thereof. Thus, the timing belt 12 is gradually restored into an original position.