A head rest adjusting device using a friction mechanism, which permits the forward and backward rotative adjustment of the head rest relative to a head rest stay. The friction mechanism comprises a substantially U-shaped leaf spring receiving the head rest stay therein, and a protruding resilient section is formed on the folded area of the leaf spring so as to give an additional resiliency to the leaf spring per se and thus provide a stable proper friction against the rotation of the head rest about the stay.