To surely prevent shaking of a door frame and its departure from a rail while facilitating fitting work of a bracing device to the door frame and its positional adjustment in relation to the rail.

The bracing device 10 is placed above the door frame 1 moving on a rail 2 of a sash. The bracing device 10 comprises a fixed member 12 fitted to the door frame 1 in a fixing manner, and a moving member 14 fitted to the fixed member 12 in a freely slidable manner. The fixed member 12 has an engaging piece 16 elastically engaged with the rail 2 from below. The moving member 14 can be fastened and fixed to the fixed body 12 at an arbitrary position in relation to the fixed body 12. A channel part 14a is formed in an upper end of the moving member 14 astride the rail 2.