A connecting device of an accessory by which improvement in mechanical strength is compatible with arrangements of many connection terminals. The connecting device is detachably attachable to an accessory shoe device of an electronic apparatus. An engagement member is attachable to the accessory shoe device by sliding in a first direction. A holding member is fixed to the engagement member by fastening members. A connecting plug has connection terminals and is arranged at a front part in the first direction of the holding member. The fastening members are arranged at positions that are spaced apart from each other in a second direction that perpendicularly intersects with the first direction. A width of the holding member is shorter than a width of the engagement member in the second direction, and the connection terminals are arranged in an area between the positions of the fastening members.