A receptacle (101, 101A, 101B, 101C, 101D, 101E) connectable to a conventional plug being compliant with a high definition multimedia interface, HDMI, specification; and connectable to a plug (102, 102A, 102B, 102C, 102D, 102E), wherein the receptacle (101, 101A, 101B, 101C, 101D, 101E) includes: - a tubular body forming a holding space that enables insertion of the conventional plug, - a first recessed part (112, 112B, 112C) forming a space that narrows heading from the holding space in an insertion direction of the plug (102, 102A, 102B, 102C, 102D, 102E), 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 (102, 102A, 102B, 102C, 102D, 102E), and wherein the plug (102, 102A, 102B, 102C, 102D, 102E) includes: - an insertion part (121, 121A, 121B, 121C, 121D) shaped to be insertable into the tubular body, - a first projection (122, 122B, 122C, 122D) projecting from the insertion part (121, 121A, 121B, 121C, 121D) in the insertion direction, the first projection (122, 122B, 122C, 122D) being capable of fitting into the first recessed part (112, 112B, 112C), 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 (112, 112B, 112C) of the receptacle (101, 101A, 101B, 101C, 101D, 101E) includes, at a bottom of the first recessed part (112, 112B, 112C), a second projection arranged to project toward the plug (102, 102A, 102B, 102C, 102D, 102E), the first end of the receptacle (101, 101A, 101B, 101C, 101D, 101E) is disposed at a projecting edge of the second projection. wherein the receptacle (101, 101A, 101B, 101C, 101D, 101E) includes: - a tubular body forming a holding space that enables insertion of the conventional plug, - a first recessed part (112, 112B, 112C) forming a space that narrows heading from the holding space in an insertion direction of the plug (102, 102A, 102B, 102C, 102D, 102E), 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 (102, 102A, 102B, 102C, 102D, 102E), and - a tubular body forming a holding space that enables insertion of the conventional plug, - a first recessed part (112, 112B, 112C) forming a space that narrows heading from the holding space in an insertion direction of the plug (102, 102A, 102B, 102C, 102D, 102E), 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 (102, 102A, 102B, 102C, 102D, 102E), and wherein the plug (102, 102A, 102B, 102C, 102D, 102E) includes: - an insertion part (121, 121A, 121B, 121C, 121D) shaped to be insertable into the tubular body, - a first projection (122, 122B, 122C, 122D) projecting from the insertion part (121, 121A, 121B, 121C, 121D) in the insertion direction, the first projection (122, 122B, 122C, 122D) being capable of fitting into the first recessed part (112, 112B, 112C), 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 (121, 121A, 121B, 121C, 121D) shaped to be insertable into the tubular body, - a first projection (122, 122B, 122C, 122D) projecting from the insertion part (121, 121A, 121B, 121C, 121D) in the insertion direction, the first projection (122, 122B, 122C, 122D) being capable of fitting into the first recessed part (112, 112B, 112C), 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 (112, 112B, 112C) of the receptacle (101, 101A, 101B, 101C, 101D, 101E) includes, at a bottom of the first recessed part (112, 112B, 112C), a second projection arranged to project toward the plug (102, 102A, 102B, 102C, 102D, 102E), - the first recessed part (112, 112B, 112C) of the receptacle (101, 101A, 101B, 101C, 101D, 101E) includes, at a bottom of the first recessed part (112, 112B, 112C), a second projection arranged to project toward the plug (102, 102A, 102B, 102C, 102D, 102E), the first end of the receptacle (101, 101A, 101B, 101C, 101D, 101E) is disposed at a projecting edge of the second projection.