The invention relates to an elevator guide rail straightness measuring system, for measuring the straightness of elevator guide rails, which measuring system comprises at least one plumb line mounted vertically in the runway adjacent to the guide rail and at least one sensor arrangement to be mounted on a carrier to travel vertically along the guide rail, which sensor arrangement comprises a frame, at least one guide shoe connected to the frame for sliding/rolling along a guide surface of the guide rail, a bias means for placing and biasing the frame against the guide surface, and at least one sensor means for sensing the position of the plumb line with respect to the frame, such elevator system allows easy and exact measurement of the guide rail straightness. The invention also relates to an elevator having such a system.