An adjusting device for longitudinal adjustment of a vehicle seat comprising a first rail and a second rail movably guided in a longitudinal direction relative to the first rail includes a guide piece mounted to a side-wall of the first rail, comprising a main body, a plurality of connecting devices for connecting to the first rail, and a positioning device locked in a positioning recess, for positioning the guide piece with respect to the first rail in the longitudinal direction. The connecting devices are formed as hook-like members that are inserted through the locking recesses and grip behind the side-wall to fix guide piece to the first rail. The guide piece comprises a flexible portion connected to the positioning device and the main body, which is configured to bend in a transverse direction when mounting the guide piece to the first rail and to be locked in the positioning recess.