A hinge device includes a coupler having two substantially C-shaped outer clamp members which are interconnected back-to-back at a juncture and which respectively have open mouths opposite to the juncture, and two substantially cylindrical inner clamp members fitted rotatably and respectively in the outer clamp members. Each inner clamp member has a substantially cylindrical outer surface, and two clamping arms defining an axially extending clamping groove therebetween. The clamping groove has a groove opening alignable with the open mouth of one of the outer clamp members. A hoop assembly having two hoops interconnected hingedly by the hinge device is also disclosed.