A bracket for attaching an actuator is easily bonded to a side frame while avoiding interference with a shaft for driving a reclining mechanism. In a seat side frame which is provided at each of both ends of a seat back frame in the width direction, a reclining mechanism, and an actuator, the reclining mechanism is attached to a side wall of the side frame, and the actuator is fixed to an attachment bracket bonded to an inner surface of a portion to which the reclining mechanism is attached in the side wall in the width direction. An upper portion which is disposed above the attachment portion of the reclining mechanism and a lower portion which is disposed below the attachment portion thereof are connected to each other at the rear ends thereof and are separated from each other at the front ends thereof.