To provide a locking structure for a sliding door device capable of locking even a sliding door installed as attached from the outer side in a simple structure.

A sliding door 5 is attached from the outer side to be installed on an opening 1 to be freely opened and closed in a sliding door device. A lock device 7 is installed on an opening frame 6 in the opening 1, and a lock bar 9 to get in and out in a thickness direction of the sliding door 5 by operation of a lever 8 is provided on the lock device 7. A through hole 10 is provided to penetrate the thickness of the sliding door 5 at a side part of the sliding door 5, a lock bar inserting metal fitting 11 is disposed on the back surface side of the through hole 10, a through hole covering plate 12 is disposed on the surface side of the through hole 10, and the lock bar inserting metal fitting 11 and the through hole covering plate 12 are coupled with each other and fixed to the sliding door 5. An unlocking means for unlocking from the surface side of the sliding door 5 as the lock bar 9 is inserted into a lock hole 13 in the lock bar inserting metal fitting 11 to be locked is provided.