An auxiliary assembly device is provided for premounting a linear guide system on a top part. The linear guide system includes two linear guides having two respective mutually displaceable guide rails, which have mutually facing raceways. Rolling elements, which are disposed in a cage, are located between the mutually facing raceways. The auxiliary assembly device has an auxiliary rail, which is configured for connection to a respective narrow end of the two linear guides in order to hold the two linear guides with the top part in a preassembled state for insertion into a bottom part. A method for premounting a linear guide system on a top part is also provided.