To provide a new opening/closing mechanism for a building opening capable of positively preventing infiltration of rainwater, powdery snow, or the like into the indoor side without complicating the structure of a sill and a lower rail.

The opening/closing mechanism for the building opening is constituted by providing the lower rail 22 of fittings 2 with elastic sealing material parts 3, 3 mounted to the lower face indoor side IS and/or outdoor side OS over the whole length to leave a slight clearance 33 between themselves and the upper face of the sill 43 only in a position where the building opening 4 is not closed with the fittings 2, while forming cut-off parts 51, 51 for settlement deep enough to allow the lower edges of the elastic sealing material parts 3, 3 to abut on the upper face of the sill 43, at the predetermined parts of the upper side of the rail 5 at which sash rollers 25, 25 are positioned when the building opening 4 is closed with the fittings 2.