It is to suppress the deformation of a tubular support. A headrest support structure includes: a stay extending downward from a headrest; a bracket provided to a backrest so as to be formed in a tubular shape, and extending vertically; a tubular portion having an insertion hole in which the stay is inserted, the tubular portion being fitted into the bracket, and extending vertically; a first projection portion projecting from an outer peripheral surface of an upper part of the tubular portion, and facing to an inner surface of the bracket; a second projection portion projecting from the outer peripheral surface of a middle part in an up-and-down direction of the tubular portion, and facing to the inner surface of the bracket; a third projection portion projecting from the outer peripheral surface of a lower part of the tubular portion, and facing to the inner surface of the bracket; a first connection portion located between the first projection portion and the second projection portion; and a second connection portion located between the second projection portion and the third projection portion.