An attachment structure for directly attaching a shield connector to an opening formed in a metal casing is provided. A shield connector for directly mounting on a device is attached to an opening formed in a metal casing. A connector housing is fastened to the casing with bolts from outside the casing. The opening of the metal casing has a diameter-enlarged portion formed in an end portion of the opening in an outside direction of the casing to be enlarged toward the outside direction. The shield shell has a contact piece at an end portion of the shield shell, and the contact piece is housed in and contacts with the diameter-enlarged portion in a state that the shield connector is attached to the opening.