Abstract: An apparatus that accurately adjusts the angle of the back of a seat that prevents the twist caused by an external impact to a vehicle to mitigate the risk of injury. The apparatus includes turning pieces, each of which are mounted to supporting brackets that are fixed to both ends of a seating portion and back of the seat. The turning pieces are connected on a connecting shaft and are turned by means of an actuating lever. The apparatus for adjusting the angle of the back includes a concave portion in an arc formed on one surface of a turning piece connected to the back of the turning pieces on the connecting shaft. A guiding protrusion having an upper end in a cylindrical form on the connecting shaft such that it is guided in the concave portion by surface contact on one surface of the turning piece connected to the seating portion of the turning pieces.