A plug-jack connecting structure is provided which includes a jack having a sleeve at a forward end of the jack. The sleeve has a non-circular outside shape and has grooves cut in an outer periphery thereof. A plug has a ring rotatably mounted thereon at one end thereof for receiving the sleeve. The ring has a sleeve receiving hole which has a shape complementary to the outside shape of the sleeve. By turning the ring through a 90 degree angle into engagement with the grooves, the jack and the plug are connected in a state in which they are prevented from being disengaged.