An insulation-piercing connector includes a connector main body made of an insulating support block having inclined bases inclined in the same direction and a reverse direction and contacts embedded therein to expose contact tails. The contact tails are arranged in first, second, third and fourth rows in parallel with and spaced from each other and staggered or shifted from each other. A cover includes inclined bases inclined in reverse directions to those of the mating inclined bases of the connector main body and receiving slots for receiving the contact tails of the connector main body. Inclined surfaces of the inclined bases of the connector main body and the cover are curved surfaces. The first and second rows and the third and fourth rows of the contact tails are arranged in symmetry with respect to tops of the curved surfaces, respectively. The inclined bases of the connector main body and the cover are provided with protrusions for guiding the connection wires.