An insulator body 310 is held in a semicylindrical metallic cover 320. The insulator body includes a main body portion having a planar plate-like contact support 312 extending forwardly from the front end of the main body portion. A plurality of narrow strip contacts 330 are arranged on the opposite plate surfaces of the support 312. Key bosses 314, 345 extend forwardly from the insulator body 310 in such a fashion as to sandwich the contact support 312 therebetween. A part of an annular groove is defined between key bosses 314, 345 and metallic cover 320 for receiving a tubular metallic cover of a connector plug. The key boss 314 has a keyway formed in the surface thereof opposing the contact support 312 The connector provides for accommodating an increased number of contacts and preventing coupling between different types of connectors, and allows for reducing the size of the connector.