One embodiment provides a retaining device. The retaining device includes a retaining member and a case member. The retaining member has a base and a shaft that protrudes from the side of the base. The case member includes: a frame which forms an opening; a pair of side walls which are erected from both side edges of the frame; and an insertion opening which is formed between the pair of side walls for inserting the retaining member from the rear side of the case member. A shaft hole is formed in each side wall, and the shaft is inserted into the shaft holes. A first slit is formed in each side wall and located between the insertion opening and the shaft hole.