PURPOSE: To secure a necessary insulation distance for preventing partial discharge or dielectric breakdown by getting only a center part of an insulation body in contact with a connection metal, and setting an insulation distance by the insulation body to be long along the length.

CONSTITUTION: An insulation body 2 comprises conical parts 3, 3 like megaphones surrounding a connection metal 1 to have the largest diameter at both end parts and be contracted at a center part. Wires 10, 10 are connected, and it is put on a floor surface 11, then an insulation distance C and a space insulation distance D are formed between the connection metal 1 and the floor surface 11. A necessary thickness of the insulation matter for preventing partial discharge or dielectric breakdown can be secured not by the thickness of the insulation body 2 but by a passage of the insulation distance C, thereby a connection device can be made compact and light weight.