Abstract: A forward end part of an arm part (53) of a seatback frame is connected to a forward end part of a main frame (3) such that the forward end part of said arm part (53) can turn about a horizontal axis (61) extending leftward and rightward. A cam hole (71) is formed in a forward end part of the arm part (53). A shaft part (72) movably engageable with the cam hole (71) through a roller (73) is disposed at a forward end part of a seat frame (41). The cam hole (71), the shaft part (72) and the roller (73) constitute a positive motion mechanism (70) for causing a seat frame (41) to be tilted in accordance with the turning motion of the arm part (53).