A driving force transmission mechanism is provided. The driving force transmission mechanism includes an input member including a concave shape portion; a rotation driving shaft; and a driving force transmission member that is configured to rotate in a rotation direction of the rotation driving shaft together with the rotation driving shaft. A protrusion is formed on a surface of a tip end portion of the driving force transmission member, the surface is opposed to the concave shape portion, the protrusion is configured to be engaged, from an inner side, with an edge of the concave shape portion when the tip end portion is in a contact with the edge of the concave shape portion and is tilted.