We propose changing-OPD (optical path difference) measurement technique which can be built in a scanning white-light interference microscope. The technique uses a laser beam introduced around an optical axis of the interference microscope. The laser beam reflected by a reference mirror in an objective lens and the laser beam reflected by a specimen surface make interferograms at an observing plane. These interferograms are continuously captured by a high-speed CCD line camera during a vertical scanning. Changing light intensities at two pixels having different phase and different modulations are normalized as having unity amplitude, and then subtraction and sum of them are calculated. From the calculated values changing phase is calculated and then change of OPD is also calculated after every interferogram capturing. To check accuracy of the proposed technique we carried out an experiment using a precision vertical scanning stage. After a 96μm vertical scanning, total changed OPD at different portions on the specimen surface are calculated and estimated having less than 10 nm in difference.