To provide a sliding door device which can positively prevent sash rollers of a sliding door from derailing from a traveling rail, is easily produced and mountable after setting of the sliding door in a sliding door frame, and facilitates mounting of the sliding door.

The sliding door device H is set up by arranging externally locking mechanisms 1 on the sliding door 4 traveling on the traveling rail 3 laid on a floor surface by the sash rollers, in a manner striding the traveling rail 3. The externally locking mechanisms 1 are each formed of plate portions 11a, 11a pinching the traveling rail 3 and extending along a longitudinal direction of the same, and fixed to the door frame 40 at locations that are on a front side of the sash roller 5 with both flanges, located on a door-leading side of the door frame 40, and on a rear side of the sash roller 5 with both the flanges, located on a door-tail side of the door frame 40.