A side sill structure of a vehicle mounted with opposite sliding doors, the vehicle including a lower rail that is mounted at a lower side of a vehicle body in a front-rear direction of the vehicle body, the side sill structure comprising: a side sill that includes a side sill inner panel and a side sill outer panel coupled to define an opened cross section, the side sill inner panel being mounted on the vehicle body and extending in the front-rear direction, and the side sill outer panel being coupled to the side sill inner panel to define a hollow portion between the side sill outer panel and the side sill inner panel; a lower rail accommodating unit that is coupled to the side sill so that the side sill is accommodated in the lower rail; and a side sill lower reinforcement that has one side that is coupled to and overlaps the lower rail accommodating unit, and another side that is coupled to and overlaps the side sill outer panel; wherein the lower rail accommodating unit comprises: an upper mounting reinforcement that is coupled to and overlaps a center floor panel and has a lower portion on which the lower rail is mounted; and a lower mounting reinforcement that is bent to accommodate the lower rail and has one side that is coupled to and overlaps the side sill outer panel, and another side that is coupled to and overlaps the upper mounting reinforcement; wherein the lower mounting reinforcement and the side sill lower reinforcement are welded in an H direction; and wherein the side sill outer panel and the side sill lower reinforcement are welded in an L direction. a side sill that includes a side sill inner panel and a side sill outer panel coupled to define an opened cross section, the side sill inner panel being mounted on the vehicle body and extending in the front-rear direction, and the side sill outer panel being coupled to the side sill inner panel to define a hollow portion between the side sill outer panel and the side sill inner panel; a lower rail accommodating unit that is coupled to the side sill so that the side sill is accommodated in the lower rail; and a side sill lower reinforcement that has one side that is coupled to and overlaps the lower rail accommodating unit, and another side that is coupled to and overlaps the side sill outer panel; wherein the lower rail accommodating unit comprises: an upper mounting reinforcement that is coupled to and overlaps a center floor panel and has a lower portion on which the lower rail is mounted; and a lower mounting reinforcement that is bent to accommodate the lower rail and has one side that is coupled to and overlaps the side sill outer panel, and another side that is coupled to and overlaps the upper mounting reinforcement; an upper mounting reinforcement that is coupled to and overlaps a center floor panel and has a lower portion on which the lower rail is mounted; and a lower mounting reinforcement that is bent to accommodate the lower rail and has one side that is coupled to and overlaps the side sill outer panel, and another side that is coupled to and overlaps the upper mounting reinforcement; wherein the lower mounting reinforcement and the side sill lower reinforcement are welded in an H direction; and wherein the side sill outer panel and the side sill lower reinforcement are welded in an L direction.