A seat frame includes a projecting portion formed on a surface of a recliner to face a reinforcement plate, the projecting portion having a cross sectional shape with a recessed portion and a protruded portion in a rotational direction of the recliner, the reinforcement plate including a first through-hole for fitting the projecting portion, the upward-downward direction frame including a second through-hole larger than the first through-hole which extends within the second through-hole when the upward-downward direction frame and the reinforcement plate are overlaid, and a three-member welding portion formed by a periphery of the projecting portion, a periphery of the first through-hole, and a periphery of the second through-hole wherein the recliner, the reinforcement plate, and the upward-downward direction frame are welded with the projecting portion of the recliner fitting to the first through-hole of the reinforcement plate while the reinforcement plate and the upward-downward direction frame are overlaid.