To provide a side sill structure which can increase the retracting amount of a slide door supported by a lower rail.

An opening part 13 extending in the longitudinal direction of a vehicle body is opened at a sill outer 14 of a side sill 4, and a sill extension 31 is formed at the opening part 13. The sill extension 31 involves an upper part retreated surface 36 which is retreated from the upper edge 35 of the opening part 13 to sill inner 15 side, and a lower part retreat surface 38 which is retreated from the lower edge 37 of the opening part 13 to the sill inner 15 side, whereas the lower part retreat surface 38 is formed with a rising surface 39 rising along a second hanging surface 19 of the sill inner 15, and a fixed surface 40 extending along the upper part retreat surface 36 from the rising surface 39. The fixed surface 40 is fixed at an extending surface 18, and the front end of a lower rail 5 is curved to such a degree that it approaches the rising surface 39 of the sill extension 31.