An adapter device includes a first mount on one side and a second mount on another side. The first mount is detachably mounted to an imaging apparatus, and the second mount is detachably mounted to an accessory. The first mount has first through third mount recesses and first through third mount claws, disposed following a circumferential direction. The second mount has fourth through sixth mount recesses and fourth through sixth mount claws, disposed following the circumferential direction. The first through third mount claws are insertable to mount recesses on another imaging apparatus. The fourth through sixth mount claws are insertable to mount recesses on another accessory. A smallest angle range out of angle ranges in the circumferential direction of the first through third mount recesses is equal to or smaller than a smallest angle range out of angle ranges in the circumferential direction of the fourth through sixth mount claws.