A rigidity reinforcing structure for B-pillarless opposite sliding doors may include a column impact beam which is vertically mounted at one side of the door to support the door in an up-down direction of a vehicle; a center rail which is horizontally mounted at a center portion of the door to support the door in a front-rear direction of the vehicle and allows the door to slide; and a lower impact beam which is horizontally mounted at a lower side of the door to support the door in the front-rear rear direction and has one end portion where an intersection portion between the lower impact beam and the column impact beam is formed.