Several means 71, 72, 73, 74, and 75 are provided for restricting a relative displacement in a separating direction of half-split housings 50L, 50R. At one or more of several screw-connection parts 60 of the left and right half-split housings 50L, 50R, a press-fitting protrusion 71a is provided in an insertion hole 62a of a boss-receiving part 62 into which a screw-boss part 61 is inserted. By press-fitting the screw-boss part 61 to the boss-receiving part 62, a separation resistance may be introduced between the left and right half-split housings 50L, 50R.