An accessory includes a main body including a communication module configured to communicate with a predetermined network, and a transmission member that includes a window exposed to an outside and configured to pass a predetermined optical signal. The main body further includes an optical signal light-emitting unit located inward of the window of the transmission member and configured to emit the predetermined optical signal in a transmission direction, and a side member defining a circumferential external appearance of the main body and including an inclined surface that slopes with respect to a central axis passing through the main body in a vertical direction and that defines a transmission member location hole configured to receive the transmission member. The window includes a central part parallel with the central axis, and the predetermined optical signal emitted from the optical signal light-emitting unit perpendicularly passes through the central part.