A seat back frame for a seat of a vehicle includes: a pair of side frames placed at a left side and a right side in a width direction of a seat; a head rest supporting frame supporting a head rest; a supporting plate, a both ends of the supporting plate being connected to a lower portion of each of the side frames; an upper opening provided above the supporting plate, the upper opening being opened in a front-rear direction; a planar elastic body provided across the upper opening; and a connection member elastically connecting a lower portion of the planar elastic body and an upper portion of the supporting plate.