Abstract: A screen-operating device for use in a roller blind having a braking mechanism consisting a brake-drum secured to a bracket, a coil spring fitted on the brake-drum, a disk loosely mounted on the coil spring and integrally coupled with a screen-roll on which a screen is wound, and a cord-pulley mounted on the disk. The coil spring has their both ends inserted in an opening in the disk and the cord-pulley has its inner tung inserted between the both ends in the opening. Torque from the cord-pulley side acts on either of the both ends to loosen the coil spring and then rotates the screen-roll to raise and lower the screen. But, any torque from the screen side acts on the end to fasten the coil spring on the brake-drum and is immediately braked.