To semi-automatically open and close a sliding door by swingably supporting a second rail inserted in the groove part of a first rail in the form of a seesaw for the first rail with a pivot as a fulcrum at an intermediate point along the length direction.

A second rail 4 is swingably supported on the groove part of a first long rail 2 in the form of a seesaw for the first rail with a pivot 5 as a fulcrum at an intermediate point along the length direction. The rail A of a sliding door B is formed. The length L1 of one end of the second rail 4 to the pivot 5 is preferably slightly shorter than the horizontal length L2 of the sliding door B running along the groove 3 of the second rail 4. Thereby when the load center of the sliding door B passes the pivot 5, the second rail 4 inclines downward with the pivot 5 as the fulcrum to the passed side, and the sliding door B can be self-run in the inclined lower order side.