The present invention provides an adapter device to which a small video camera can easily be attached. The present invention provides an adapter device to which a video camera device is to be attached, the adapter device including an adapter body having formed therein a receptacle in which the video camera device is to be installed and having formed at the rear end thereof an opening through which the video camera device is to be inserted, a mechanical connecting mechanism to be connected to the receptacle in the position of a lens-unit mount of the video camera device, and an electrical connecting mechanism to electrically connect the receptacle and video camera device, the video camera device being to be inserted first at the lens-unit mount thereof into the receptacle from the insertion opening at the rear end of the adapter body, the mechanical connecting mechanism being provided at the front end of the adapter body, and the electrical connecting mechanism being provided on the bottom of the receptacle to receive the terminal block of the video camera device.