Two springs inserted in series, which press a tension roller against a drive belt, are provided in a tension device for a drive belt. The tension spring essentially effects adjustment of the tension roller during assembly and after that effectively is inoperative. The working spring is active during operation and within a limited range of adjustment of the tension roller. The tensions of the two springs are designed so that during assembly equilibrium of forces exists at a middle working point within the path of adjustment. These features result in a simple and precise adjustment during assembly.