To provide a double sliding sash effective in preventing break-in theft damage by mounting an auxiliary lock of simple structure in addition to a crescent lock so that it takes much time to break in.

The double sliding sash is provided with the auxiliary lock, and a meeting style of an outer sash has a receiving groove opened on the inner sash meeting style side, over the longitudinal direction of the outer sash meeting style. The auxiliary lock is provided with a dead bolt, an operating member and an elastic member. The dead bolt can project and recede into the receiving groove of the outer sash meeting style from between a smoke guard piece of the inner sash meeting style and a tight material holder part. The elastic member energizes the dead bolt in a receding direction into the inner sash meeting style, and the operating member is located almost at the depth center of an indoor side face of the inner sash meeting style to operate the dead bolt to project/recede.