In electric equipment that has a digital interface, an operation that is normally performed by an output terminal during authentication processing is executed on equipment connected to an input terminal of the digital interface by the input terminal as an erroneous connection detection operation. When there is a proper response from the equipment connected to the input terminal of the digital interface in response to the erroneous connection detection operation, the electric equipment detects that the connected equipment has been erroneously connected. When an erroneous connection is detected, the electric equipment gives notice of the detection of the erroneous connection.