A connecting device which has an excellent appearance, produces no frictional sound between a connecting device main body such as a swivel hook main body and a belt attaching member and can be assembled easily and securely. The connecting device includes a neck portion (24) standing from a proximal portion (22-1) of a main body (2, 50) of the connecting device (1, 5) and an engagement head (25) which is expanded in an outward direction form a center of an axis line such that a diameter thereof can be reduced, the engagement head being provided at a front end of the neck portion (24). A belt attaching member is connected to the main body rotatably around the neck portion. A through hole which the engagement head can be engaged with by insertion is formed in part of the belt attaching member. In the through hole, a small-diameter hole portion having a smaller diameter than the diameter of the engagement head and a large-diameter hole portion having a large diameter than the diameter of the engagement head are connected via a stepped portion. The large-diameter hole portion has a ring-like member which allows the neck portion to be inserted therethrough. The ring-like member has a cylindrical wall portion which is accommodated in the large-diameter hole portion and a ring-like engaging portion provided at an end of the wall portion and extended toward a center thereof. An inside diameter of the engaging portion (43) is equal to an outside diameter of the diameter of the engagement head (25) when it is reduced.