A seat slide locking apparatus comprises a lower rail provided with a plurality of locking holes, and a plurality of locking pieces provided on the lower rail. The locking pieces include a first locking piece 4a having a broad width inserting section 46 and a second locking piece 4b having a narrow width inserting section 47, and are held by a holding member which has a holding hole 66. The holding hole 66 includes a broad width section 66a into which the broad width inserting section 46 is inserted and a narrow width section 66b into which the narrow width inserting section 47 is inserted.