To provide a frame body attachment/detachment structure suitable for reducing the size and thickness of a detachable part, facilitating attachment and detachment operations, and capable of providing a sure engagement state.

An inner frame body 4 is provided with a base part 44 rising from a peripheral part, and engagement pieces 45 circumferentially extending from the base part 44. An outer frame body 2 is provided with engagement projections 51 projecting inward; one-side surface of each of them is formed into a gently slanting recessed surface continuing to the inside surface of the outer frame body 2; the curvature of an entrance-side half part 52 of the recessed surface is set smaller than that of a deep-side half part 53 to form a circumferential step 54 between the entrance-side half part 52 and the deep-side half part 53; and the inside surface of the entrance-side half part 52 is formed into a tapered surface gently slantingly continuing to the inside surface of the outer frame body 2. When the inner frame body 4 is pushed into the recessed surface of the outer frame body 2 by matching the engagement piece 45 of the inner frame body 4 to it, the engagement piece 45 is engaged with the inner frame body by overriding the step 54 formed between the entrance-side half part 52 and the deep-side half part 53 while bending inward along the entrance-side half part 52 of the engagement projection 51.