Abstract: A swingable connecting device includes an oscillation supporting mechanism for swingably supporting a shackle to a supporting body. A mounting bolt of the supporting body has a first male threaded portion and a second male threaded portion formed separately at opposite ends thereof with different diameters. A bolt shaft portion whose diameter varies continuously from the first male threaded portion to the second male threaded portion is formed between the first male threaded portion and the second male threaded portion. The supporting body is provided with a bolt hole for allowing the bolt shaft portion to fit into and engage with the bolt bore, through which the small-diameter second male threaded portion extends. The bolt bore has an inner surface profile, in which an inner diameter thereof continuously varies with an outer circumferential surface of the bolt shaft portion. A nut integrally connects the mounting bolt with the supporting body.