Characteristics of errors which might occur in the straightness measurement method due to sequential-two-points were investigated. The investigation made it clear that the slight discrepancies of alignment at the tip between two displacement sensors would be accumulated onto the portion of linear increment generally observed when the straightness is measured; the method of compensation is analytically shown. It is theoretically proposed that the error which might be introduced by the yaw of the tool post and is neglected by the present system can be evaluated by the measurement using sequential-three-points. It is also shown that the measurement system due to the sequential-two-points works stably even for the case when the work on the lathe is rotated. The cylindricity obtained by the measurement during the work rotation agrees well with that obtained by static measurements.