Provided is a simple and compact headrest support device. The headrest support device (20) comprises a support member (22) coupled to a frame (2) of a seatback (1), and a headrest pillar structure including a pillar base portion (25) extending laterally and supported by the support member so as to be rotatable around an axial line of the pillar base portion, and a pair of pillar main body portions (26) extending substantially vertically from the pillar base portion substantially in parallel to each other in a laterally spaced apart relationship and connected to a headrest (24), wherein the support member is coupled to the frame at at least one coupling portion (34) disposed between the two pillar main body portions in front view in a headrest upright condition where the headrest is positioned above the seatback.