To provide a belt drive mechanism capable of stably driving a belt over a long period while suppressing the friction vibration.

The belt drive mechanism 110 comprises a pulley 12 rotatably supported, and the belt 13 wound on the pulley 12 for travelling with the rotation of the pulley 12. Herein, a friction adjusting means is provided for adjusting friction force working between the belt 13 and the pulley 12 during the travel of the belt 13. The friction adjusting means consists of auxiliary rolls 14, 15 rotatably supported. The auxiliary rolls 14, 15 energize the surface of the belt 13 so that the cross section of a contact portion of the belt 13 with the pulley 12, perpendicular to the travelling direction, approximates a linear shape.