Abstract: A gasket securing device of a gasket mounting appliance is movable in its radius direction, capable of being put into an opening disposed in an object after mounting a ribbonlike gasket in its backward position, and, in its forward position, capable of pressing predetermined parts of the gasket against and fitting them into respective predetermined parts of an engaging portion prepared along the margin of the opening and, thereafter, pressing the rest of the gasket that lies between the fitted predetermined parts against and fitting it into the rest of the engaging portion.