Straightness measurement is a very important technique in the field of mechanical engineering. A particular application would be very high accuracy machining on a diamond turning lathe. In this paper a novel optical method for measuring straightness of motion using reflection confocal optics is proposed. The advantage of this method in comparison with the transmission optical systems used by Burch and Williams is that the lateral displacements in the direction of the two axes perpendicular to the optical axis, and the rotation angles around all three axes, can be measured simultaneously. This can be very useful for straightness measurement. Also, reflection optical systems are more compact in length.