There is provided a head rest moving mechanism of moving a support portion of a head rest relative to a seat back when a vehicle is subjected to a back collision. The head rest moving mechanism moves the support portion from an initial position to a collision corresponding position in a front upper direction. The support portion draws a locus of moving to an upper side while an upper portion thereof is inclining to rise to a front side, successively, moving to the upper side by rising to return to an attitude thereof when the support portion is disposed at an initial position while maintaining a collision point of the support portion and the head. The head rest moving mechanism includes a base installed at an upper portion of the seat back, link means for linking the support portion and the base, an engaging pin moved integrally with the support portion for operating an attitude of the support portion in accordance with a relative movement thereof, and a guide path slidably engaged with the engaging pin for guiding to slide the engaging pin in accordance with the relative movement of the support portion. The guide path is a long hole formed at a side face portion of the base. The link means is pivotable centering on a position thereof axially supported by the base. When the vehicle is subjected to back collision, the link means is pivoted from a lower side in the front upper direction by drawing a locus in a shape of a circular arc bent to the lower side. On the other hand, the guide path is formed in a shape of a circular arc bent to the upper side extended from the initial position in the front upper direction.