PURPOSE: To prevent stumbles by providing guide rails to the insides of recessed channels of sills so that the upper ends of the guide rails are not projected from the upper surfaces of the sills and, at the same time, providing weep holes in the recessed channels, and providing water keeping channels to lower positions of the sills.

CONSTITUTION: Water keeping channels 4 are provided to a floor FL. Recessed channels 2a provided with guide rails 5 for guiding the traveling of pulleys 3 of door bodies 1 inside the channels 2a are provided to the sills 2 and, at the same time, weep holes 2c are provided in the recessed channels 2a. Openings of the water keeping channels 4 are convered with the sills 2. In addition, the guide rails 5 are so loaded to loading channels 2b of the inside of a pair of recessed channels 2a provided to the sills 2 that the upper ends of the guide rails are not projected from the upper surfaces of the sills 2.