PURPOSE: To provide a falling-off preventive lock mechanism by which a user is not struck by unexpected trouble due to unexpected external force even with the intention of being reliably installed when an accessory part or the like is installed on a camera by inserting a locking plate into an eyecup installing groove from both sides of a plate part.

CONSTITUTION: When a connecting shoe 8 of an accessory shoe adapter is inserted into a hot shoe 7 of a camera, a notch part formed on a plate part 1 is fixed to the camera side in the shape of striding over a camera eyepiece part frame 2. In order to perfectly lock this condition, when a locking plate is inserted into an eyecup installing groove 3 of the camera, the accessory shoe adapter is perfectly fixed to the camera. In this way, when the locking plate is inserted into the eyecup installing groove 3 from a side surface, the accessory shoe adapter is fixed without being carelessly dislocated to the rear of the camera.