A lumbar support of a car seat according to the present invention includes: a support plate to support user's back and made of elastic material; a support frame to which the support plate is rotatably engaged; a slider member mounted on the support frame in a movable manner in a vertical direction; a vertically driving means to move the slider member on the support frame; a biasing force control means including an upper member moving together with the slider member, a lower member disposed below the upper member, a biasing member connecting the upper and lower members to each other and having a controllable biasing force acting on the support plate depending on a distance between the upper and lower members, and an actuating means to control the distance between the upper member and the lower member.