To prevent malfunction of a lock caused by accidental triggering.

A sliding door device includes first detection means and second detection means for detecting a fully closed state of a door body 1. The first detection means includes a trigger 10 that protrudes from a depth face of a door-end-side vertical frame 5. Upon operation of the trigger 10, a first detection switch is switched on to detect the fully closed state of the door body 1. The second detection means includes a second detection switch provided inside an upper frame 2 above an opening. When the door body 1 is brought into the fully closed position, the second detection switch is switched on to detect the fully closed state of the door body. Only when both the first detection switch and the second detection switch are switched on, it is determined that the door body 1 is in the fully closed state. Then a lock piece at an unlock position is moved to a lock position so as to be engaged with an engaged portion on the end surface at the door end side of the door body 1.