A cord retaining device for non-cord Venetian blinds include two sheltering caps, two snap members, two fixing seats, and two double-sided adhesive pieces wherein the sheltering cap has a registration cavity defined at the bottom side therein, and a through hole disposed at the top thereon. The snap member, shaped in form conforming to that of the registration cavity of the sheltering cap, has a support piece with a cord passage protruding at the top thereon, and an arc-shaped retaining cavity with an outwardly extended guide arc indented at the bottom side therein. The fixing seat has a solid coupling ball block extending at the top surface of a base thereon in match to the retaining cavity thereof, and the double-sided adhesive piece is shaped like the base of the fixing seat thereof. In assembly, retaining cords are tied up to the support pieces of the snap members by the bottom ends thereof before the snap members are registered with the ball blocks of the fixing seats that are mounted on top of the double-sided adhesive pieces fastened to both lateral sides of a windowsill. Thus, in case children curiously extending their heads out of the windowsill get caught by the retaining cords, the retaining cords pulled by the children struggling to get free there-from will draw upwards the snap members therewith and detach the retaining cavities thereof off from the ball blocks of the fixing seats in the first moment of emergency, efficiently preventing children from getting cut or strangled by the retaining cords to protect the safety of the family.