A seat slide locking apparatus, comprising: an elongate-shaped lower rail which is fixed to a vehicle body with a lengthwise direction aligned to a front-to-rear direction of the vehicle body, the lower rail comprising a left-side wall and a right-side wall which are arranged in mutually opposing fashion at a distance apart in a right and left direction; an elongate-shaped upper rail which is fixed to a seat with a lengthwise direction aligned to the front-rear direction of the vehicle body; a plurality of open engaging sections (14, 15) provided in the lengthwise direction of one of the lower rail and the upper rail; a holding member which is formed with a holding hole and is installed on the other one of the lower rail and the upper rail; and a plurality of locking pieces which are inserted into the holding hole of the holding member and which can enter into and exit from the open engaging sections (14, 15), wherein the plurality of locking pieces include: a first locking piece having a broad width inserting section; and characterised by a second locking piece having a narrow width inserting section with a width narrower in the right and left direction than the broad width inserting section, the first locking piece and the second locking piece being arranged in one row in such a manner that the respective thickness directions thereof in the lengthwise direction of the other one of the lower rail and the upper rail are aligned with the lengthwise direction of the other one of the lower rail and the upper rail, and the holding hole including a broad width section into which the broad width inserting section is inserted, and a narrow width section which has a width narrower in the right and left direction than the broad width section and into which the narrow width inserting section is inserted. an elongate-shaped lower rail which is fixed to a vehicle body with a lengthwise direction aligned to a front-to-rear direction of the vehicle body, the lower rail comprising a left-side wall and a right-side wall which are arranged in mutually opposing fashion at a distance apart in a right and left direction; an elongate-shaped upper rail which is fixed to a seat with a lengthwise direction aligned to the front-rear direction of the vehicle body; a plurality of open engaging sections (14, 15) provided in the lengthwise direction of one of the lower rail and the upper rail; a holding member which is formed with a holding hole and is installed on the other one of the lower rail and the upper rail; and a plurality of locking pieces which are inserted into the holding hole of the holding member and which can enter into and exit from the open engaging sections (14, 15), wherein the plurality of locking pieces include: a first locking piece having a broad width inserting section; and characterised by a second locking piece having a narrow width inserting section with a width narrower in the right and left direction than the broad width inserting section, the first locking piece and the second locking piece being arranged in one row in such a manner that the respective thickness directions thereof in the lengthwise direction of the other one of the lower rail and the upper rail are aligned with the lengthwise direction of the other one of the lower rail and the upper rail, and the holding hole including a broad width section into which the broad width inserting section is inserted, and a narrow width section which has a width narrower in the right and left direction than the broad width section and into which the narrow width inserting section is inserted. a first locking piece having a broad width inserting section; and characterised by a second locking piece having a narrow width inserting section with a width narrower in the right and left direction than the broad width inserting section, the first locking piece and the second locking piece being arranged in one row in such a manner that the respective thickness directions thereof in the lengthwise direction of the other one of the lower rail and the upper rail are aligned with the lengthwise direction of the other one of the lower rail and the upper rail, and the holding hole including a broad width section into which the broad width inserting section is inserted, and a narrow width section which has a width narrower in the right and left direction than the broad width section and into which the narrow width inserting section is inserted.