To provide a horizontally double sliding sash window with an inexpensive auxiliary lock, which is effective in the prevention of intrusion, because the auxiliary lock is invisible from outside, making it difficult to know even about its existence.

The meeting frame of the inner screen has a hollow at the outer opposite side of its panel fitting groove, and a lock mounting seat between the panel fitting groove and the hollow at a position opposite to the meeting frame of the outer screen. The meeting frame of the outer screen has a lock receiver at a position opposite to the lock mounting seat on the meeting frame of the inner screen. An auxiliary lock consists of a mount, a lock rotator and an operation lever. The mount is installed at a specified position on the meeting frame of the inner screen. The rotator and the operation lever are pivotally mounted on the mount. The operation lever is located within the hollow, and a notch is provided within the indoor wall of the hollow for the operation of the lever from indoor side. The lock rotator can be moved retractably to the lock receiver by the operation of the lever.