When trace image data is obtained while a probe is used to scan a region on a sample in a forward direction and retrace image data is obtained while the same region is scanned in the reverse direction, a deviation information storage unit stores deviation detected by a deviation detection unit. This deviation is an indication of the difference between the distance between the probe and the sample and a target value for the distance at a given point in time. An image data selection unit compares the deviation during forward scanning and the deviation during reverse scanning for each measurement point, selects the image data obtained during scanning that has the smaller deviation, and stores the same to a storage region of an image data storage unit as selected image data.