A locking structure for a rectilinear center rail is provided. The locking structure includes a rectilinear center rail mounted on a sliding door, a center roller unit rollably connected to the center rail, and a center swing arm rotatably connected to the center roller unit and a vehicle body. The center roller unit includes a first lever and a second lever that are configured to rotate about rotation axes formed in a width direction of the center rail. The locking structure is configured to switch between a first posture in which the first lever is configured to be caught by a catching portion formed on the center rail, rotated, and then locked with the second lever and a second posture in which the second lever is configured to be caught by a catching portion formed on the center swing arm, rotated, and then locked with the first lever.