A seat adjustment device adjusts the pivoting angle formed by a first seat member and a second seat member. A hinge shaft connects at a first connection point, the first seat member, and the second seat member. An arm section and a connection mechanism are connected at a second connection point. The positions of the axes of the first connection point, the second connection point, and a pivoting section are set in such a manner that, when a load is applied to the front end of either the first seat member or the second seat member, the distance from the second connection point to the axis of the pivoting section is less than the distance from the first connection point to the second connection point with respect to the direction perpendicular to the line of action of a load applied to the second connection point.