Apparatus of adjusting back protrusion of seat

The present invention relates to a seat backrest adjustment device. The present invention provides a seat backrest protrusion amount adjustment device including a curved plate positioned inside a seat backrest portion as a plate material which can be curved and deformed by elasticity; a guide member which guides a movement of the curved plate along a predetermined path; a first moving member fixated to the curved plate and moved along the guide member; a second moving member fixated to the curved plate at a position apart from the first moving member, and moved along the guide member; a moving driving screw shaft where a male screw portion engaged with a female screw portion formed in the first moving member is formed; a moving driving motor position-fixed to the guide member, drives the moving driving screw shaft to rotate, and moves the second moving member vertically to the guide member; a curved driving wire fixated to a central portion of the first moving member, extends upwards or downwards, and extends to the left or the right from the curved plate via a direction conversion pulley; and a curved driving disposed to be supported by the first moving member and pulls or releases the curved driving wire. According to the present invention, twisting of both sides of a supporting plate due to a height difference can be prevented, and thus a user can feel comfortable with a balanced operation during the course of the control amount of a seat backrest protrusion.