A self-locking step-by-step mechanism for an adjustment device of a vehicle seat has a clamping roller lock and a step-by-step device. The clamping roller lock a rotatable release wheel and a toothing. The step-by-step device has a) a pivotably mounted actuating lever, b) a driver pivotably mounted on the actuating lever and which has two driver regions which interact with the toothing of the release wheel, c) a drag lever which has supporting surfaces which enter into contact with the driver, and d) a spring. The spring has at least one projection. The drag lever has at least one indentation which is normally in engagement with the projection. The spring is fixed against rotating.