To provide an assembly unit for a toy capable of storing unused engaging section in a part unit thereof. An arm part (13) of the toy comprises a rotating member (50) rotating about a rotating shaft (55) provided in the body (14) thereof. The rotating member (50) has an engaging section (53), which is used for connecting the rotating member to a second part unit (23), which is connected to the arm part (13), and a hand (necessary member) (51). When a separate member (23) is connected to the arm part (13), the hand 51 is stored in the body 14 and the engaging section (53) is exposed to the outside of the body (14). When the separate member (23) is not connected to the arm part (13), the engaging section (53) is stored in the body (14) and the hand (51) is exposed to the outside of the body (14).