A seat recliner for a vehicle includes a step inserted in an assembly hole of a seat frame and a second flange, and a conventional sub-guide protrusion of a lock gear is removed. The second flange is provided with both a free section and a locking section using only a guide protrusion. The seat recliner includes: a first flange having a plurality of guide parts, and a guide space; a plurality of lock gears disposed in the guide spaces of the first flange; a lock cam disposed on the first flange; a control part disposed on the lock gears and the lock cam; and a second flange including a first part and the second part, and coupled to the seat frame by insertion of an outer circumferential surface of the second part into an assembly hole of the seat frame.