a jack having enhanced reliability and strength against torsional forces is provided . the jack including a plug inlet for receiving a plug having at least one electrode formed thereon for electrical signals , comprises an elongated spring having a free end part with a pressing portion capable of pressing a side surface of the inserted plug , and a proximal end part fixed to the jack . the jack further comprises a retaining portion capable of contacting a first contact portion and a second contact portion arranged at opposite sides thereof along a direction of inserting the plug when the plug is inserted into the jack .