A first housing of a first connector being fitted to a second housing of a second connector includes first and second rotation check lock portions. A third housing of the first connector includes rotation rail portions with first and second rotation prevention walls. The first rotation prevention wall allows rotation of the first rotation check lock portion beyond the first rotation prevention wall in one rotational direction, and does not allow the rotation beyond the first rotation prevention wall in the other rotational direction to blocks the rotation at a rotational position of 180 degrees or more. The second rotation prevention wall does not allow rotation of the second rotation check lock portion beyond the second rotation prevention wall in the one rotational direction to block the rotation at a rotational position of 180 degrees, and allows the rotation beyond the second rotation prevention wall in the other rotational direction.