A long slide rail device for vehicle seat, which includes a temporary lock mechanism for temporarily locking an upper rail member fixed to the seat, at a predetermined temporary lock point, relative to a long lower rail member fixed in a floor of vehicle, so that the seat may be temporarily locked at that point, allowing its seat cushion to be folded upright to its seat back, before being displaced to a storage lock position. The temporary lock mechanism includes a female lock element (or female lock hole) provided in the lower rail member at the temporary lock point, and a male lock element so arranged at the upper rail element as to lie generally within the long lower rail member and movable for engagement with and disengagement from the female lock element. Thus, the structure of long slide rail device is simplified in structure and made small in sizes. Also, the temporary lock mechanism is provided with a connecting portion for allowing operative connection with other locking mechanism for locking the seat cushion or seat back at the temporary lock point in an interlocking relation therewith.