Abstract: A safety device is disclosed for converting the inside cylinder of a double cylinder deadbolt assembly to one which captivates a key inserted into the assembly to permit it to rotate to operate the deadbolt. The device prevents removal of the inside key under normal circumstances. This is accomplished by a base member, which is attached to the face of the inside cylinder or cylinder lock assembly. The base member has a circular opening in it, defined by an inwardly-turned shoulder. A key-holding member is rotatably captured between the shoulder of the base member and the face of the cylinder lock assembly, when it is secured to the face of the lock assembly, to permit insertion and rotation of a key in the lock assembly. The key-holding member includes a device for securing the key against removal from it when the assembly is in place. No modification of the cylinder lock assembly is necessary.