The invention relates to a seat longitudinal adjusting device. The seat longitudinal adjusting device at least comprises two guide rail pairs and a movable locking member. The two guide rail pairs arearranged in a spaced manner and are each composed of two guide rails, namely an upper guide rail and a lower guide rail correspondingly. Each upper guide rail is provided with an opening, and each lower guide rail is provided with a through hole. The movable locking member is loaded with at least one spring and is fixedly held on the upper guide rails. The moving of the upper guide rails in the lower guide rails is stopped by the locking member at a locking position. A tooth-shaped protruding part can be borne on two opposite longitudinal sides of the locking member. The tooth-shaped protruding part can move from a release position to the locking position, namely, the opening and the through hole. The upper guide rails and the lower guide rails are fixed in the locking position by the locking member. The opening and/or the through hole is provided with a contact area used for the tooth-shaped protruding part of the locking member. The tooth-shaped protruding part is at least partiallyprovided with bottom cutting parts at two tooth side surfaces. An extended edge created by the bottom cutting parts is opposite to or makes contact with the contact area at the corresponding tooth side surface.