A pivot shaft mechanism includes a rotation support cylinder; an inner shaft provided in the rotation support cylinder to be relatively rotatable; an annular space formed between an inner peripheral surface of the rotation support cylinder and an outer peripheral surface of the inner shaft; engaging recesses formed on the outer peripheral surface of the inner shaft; and a resilient ring member which includes an arc-shaped arm portion which is formed as a partial annular ring, and a pair of bent end portions. The resilient ring member is installed between the rotation support cylinder and the inner shaft with the arc-shaped arm portion installed in the annular space in a resiliently deformed state and the pair of bent end portions installed in the engaging recesses in a resiliently deformed state to apply a frictional resistance between the rotation support cylinder and the inner shaft.