Abstract: Disclosed is a cap plug for a balloon which includes a body mounted on a mouth portion of a balloon, an attachment connected to the body through a detachable connecting means and a yarn for connecting the body and the attachment. The connecting means has four outwardly directed engaging pawls provided on the body and an engaging groove provided in the attachment and engageable with the engaging pawls. The desired connection is achieved by moving each of the engaging pawls into the corresponding engaging groove, and relatively rotating the attachment and the body to bring the engaging pawl into firm engagement with the engaging groove. The engagement between the body and the attachment is released by rotation thereof in a direction opposite to that when they are connected.