PURPOSE: To keep a window frame and a sash airtight using a small force by mounting attachments inside rails constituting the rail assembly of the sash, in such a manner that the attachments are opposed to frames constituting the window frame and are freely movable indoors and outdoors.

CONSTITUTION: A window frame A has a head 1 and a sill 2 and a sash B is built into the window frame A in such a way as to freely travel horizontally. An upper attachment 80 is mounted inside the upper rail 10 of the sash B in such a way as to freely move indoors and outdoors along a groove 53 and an upper projecting piece 54, and a lower attachment 81 is mounted inside the lower rail 11 of the sash B in such a way as to freely move indoors and outdoors along a groove 63 and a lower projecting piece 64. When the attachments 80, 81 are moved indoors, their respective stopping surfaces 80b, 81b are pressed against AT members to keep the window frame A and the sash B airtight.