A structure can provide separation strength of an opposed sliding door. The structure includes a rail configured to be mounted to a vehicle body in a longitudinal direction thereof, a first bracket configured to be fixedly connected to the vehicle body at a first side of the rail and having a hook receiver, a slider inserted into the rail and coupled thereto, a swing arm configured rotatably connected to a sliding door and to the slider, and a second bracket fixedly connected to the swing arm. The second bracket has on a first side thereof a hook to be received in the hook receiver. The hook is received in the hook receiver when a load is applied to the sliding door in a direction from an inside to an outside of the vehicle body with the sliding door being closed.