A headrest support structure has an upper panel, a collar and a sleeve. The upper panel extends in a width direction of a seatback frame at the upper part. The upper panel has a sleeve support part including upper and lower wall parts facing in a direction perpendicular to the width direction. The sleeve support part is in a U-shape open to a side opposite to a seating side in a longitudinal sectional shape. The upper and lower wall parts have upper and lower holes. The collar is in a tubular shape with a through hole. The collar is arranged between the upper wall part and the lower wall part to have the through hole communicate with the upper and lower holes. The sleeve is inserted through the upper hole, the through hole, and the lower hole. A rod-like leg portion of a headrest is insertable in the sleeve.