To provide a double sliding window which disables the movement of a lower part of an outer sash to an outdoor side, even if the outer sash is lifted by prying it with a tool such as a claw bar.

This double sliding window is equipped with a window frame 10, the inner sash 20 and the outer sash 30; and a member 40 for preventing disengagement of the sash, which is equipped with a movable member 41, an operation member 42 and a holding member 43, is attached to an indoor side part of a bottom rail 32 of the outer sash 30. When the operation member 42 is held by the holding member 43, a lower end of the movable member 41 is separated from a top surface 12b of a sill 12. When the holding of the operation member 42 is released, the lower end of the movable member 41 is brought into contact with the top surface 12b of the sill 12. When the outer sash 30 is lifted, the movable member 41 is relatively moved downward so as to be brought into continuous contact with the top surface 12b of the sill 12. The movable member 41 is positioned on an indoor side with respect to a rail 12a of the sill 12, so that the lower part of the outer sash 30 can be inhibited from moving to the outdoor side.