A structure of a headrest mounted on a seat, wherein the headrest has, provided at the bottom surface thereof, an under garnish made of a hard synthetic resin material and wherein a stay of the headrest is partly supported in a holder having a head portion projecting from a top portion of a seat back of the seat, the top portion of the seat back being formed by a foam padding and a trim cover assembly overlaying the foam padding. The under garnish is provided with at least one protruding portion which protrudes downwardly therefrom. This at least one protruding portion is disposed at a location oriented towards a padding provided in the seat back. A degree to which the protruding portion protrudes from the under garnish is greater than a height of the head portion of the holder.