A receptacle connectable to a conventional plug being compliant with a high definition multimedia interface, HDMI, specification; and connectable to a plug , wherein the receptacle includes: - a tubular body forming a holding space that enables insertion of the conventional plug , - a first recessed part forming a space that narrows heading from the holding space in an insertion direction of the plug , and - a first transmission body having, as an end of an optical transmission line, a first end that is disposed to be exposed to the inserted plug , and wherein the plug includes : - an insertion part shaped to be insertable into the tubular body , - a first projection projecting from the insertion part in the insertion direction, the first projection being capable of fitting into the first recessed part , and - a second transmission body having, as an end of an optical transmission line, a second end that is arranged to be disposed to face the first end , characterized in that : - the first recessed part of the receptacle includes, at a bottom of the first recessed part , a second projection arranged to project toward the plug , the first end of the receptacle is disposed at a projecting edge of the second projection . wherein the receptacle includes: - a tubular body forming a holding space that enables insertion of the conventional plug , - a first recessed part forming a space that narrows heading from the holding space in an insertion direction of the plug , and - a first transmission body having, as an end of an optical transmission line, a first end that is disposed to be exposed to the inserted plug , and - a tubular body forming a holding space that enables insertion of the conventional plug , - a first recessed part forming a space that narrows heading from the holding space in an insertion direction of the plug , and - a first transmission body having, as an end of an optical transmission line, a first end that is disposed to be exposed to the inserted plug , and wherein the plug includes : - an insertion part shaped to be insertable into the tubular body , - a first projection projecting from the insertion part in the insertion direction, the first projection being capable of fitting into the first recessed part , and - a second transmission body having, as an end of an optical transmission line, a second end that is arranged to be disposed to face the first end , - an insertion part shaped to be insertable into the tubular body , - a first projection projecting from the insertion part in the insertion direction, the first projection being capable of fitting into the first recessed part , and - a second transmission body having, as an end of an optical transmission line, a second end that is arranged to be disposed to face the first end , characterized in that : - the first recessed part of the receptacle includes, at a bottom of the first recessed part , a second projection arranged to project toward the plug , - the first recessed part of the receptacle includes, at a bottom of the first recessed part , a second projection arranged to project toward the plug , the first end of the receptacle is disposed at a projecting edge of the second projection . <EOS>