A resilient leaning position-restoring device for a height-adjustable chair back of an office chair includes a back support unit, a height-adjustable chair back disposed on the back support unit, and a resilient leaning position-restoring device disposed between the back support unit and the chair back. The resilient leaning position-restoring device essentially includes a sliding element slidable along a slot disposed on the back support unit and being pulled downward by a spring. The upper end of the sliding element extends to form a resilient member of an inverted U shape. A lower end of the resilient member is coupled to the chair back. The resilient member maintains the predetermined resilience thereof for supporting the bottom of the chair back while height adjustment of the chair back is underway. The bottom of the chair back lends proper support to the sitter's waist, regardless of an adjusted position of the chair back.