PURPOSE: To improve the movability of a wheelchair or the like and a storm water- resistant property, by a method wherein levels of a rising wall of a basic sill to be attached to a housing frame, a covering member for an outdoorside drainage ditch, and an outer sash-guiding rail are almost matched with each other, and the rising wall is made high.

CONSTITUTION: The height of an indoorside rising wall 14 of a basic sill 7 is made high, and storm water gathering among a preventive member part 24 against falling-in of a wheel chair, a horinzontal wall 13 and an intermediate rising wall 15 is prevented from leaking into an indoorside floor F, by the rising wall 14. On the other hand, the floor F, an inner sash-guiding rail 11, a support rail 53, and a grating 51 for a drainage ditch are positioned on the same level with each other, and hence the wheelchair or the like can be smoothly moved into or out of a room. Small-sized wheels 26 are prevented from falling in the sill 7, by the preventive member part 24 or an installation strip part 23. An auxiliary sill 45, which is made removable, is removed, and a draining opening S blocked with dust is cleaned. It is unnecessary to additionally form a drain port on the sills 7, 45, by the preformation of the drain opening S. In this way, the wheelchair or the like can be smoothly moved into or out of the room, and the storm water can be prevented from leaking into the room.