To provide a device for detecting and discriminating minute change in surface property like an oxidized film.

This device is a device for discriminating the change of the inside surface property of a steel tube 12 and provided with an image pickup unit 13 for imaging the inside surface of the steel tube 12, a driving device 14 for driving the image pickup unit 13, a drive controller 15 for controlling the driving device 14 and adjusting the relative positions of the image pickup unit 13 and the steel tube 12, an image processor 16 in which the picture imaged with the image pickup unit 13 is inputted and with which a surface property discriminating value for discriminating the change of the surface property is obtained by processing the picture and a surface property discriminator 17 for connecting the surface property discriminating value inputted from the image processor 16 to the relative position information inputted from the drive controller 15. The picture processing in the image processor 16 is for determining a feature value from the gray level histogram of the picture and determining the surface property discriminating value by comparing the determined feature value with a preset threshold value. In this way, the change in the surface property caused by the oxidized film which is generated on the inside surface of the steel tube is also accurately discriminated over the entire length of the steel tube.