A slide locking apparatus has: a first locking piece having a narrow width locking hook section, a third locking piece having a broad width locking hook section and a second locking hook having an intermediate width locking hook section, which are held on an upper aril; and first to third locking piece accommodating holes which have width corresponding to the widths of the respective locking hook sections of the first locking piece to the third locking piece, and which are provided in a lower rail. The first locking piece accommodating hole is disposed on a frontmost end, the second locking piece accommodating hole is disposed to the rear side of the first locking piece accommodating hole, and the third locking piece accommodating holes are disposed to the rear side of the second locking piece accommodating hole.