PURPOSE: To make the automation of measurement posible by constituting a titled device of a discoid stage for rotating the covering material for wire rods, a line scanning type solid state image sensing element disposed perpendicularly to the rotating direction, and a circuit of an element for intensifying the change in the rotating direction of the material in the direction perpendicular to said rotating direction.

CONSTITUTION: A sample 1 is rotated by an automatic stage 7. The sample is made vertically movable as well in order to measure different parts of the focal position. A line scanning type solid state image sensing element 4 reads the sample 1 through an optical system 6 and outputs the video information corresponding to the brightness thereof. In a measuring circuit 9, the analog information outputted from the element 4 is digitized and further a space filter for detecting the data in the direction of the deteriorating parts from the data scanned forward and backward is used; thereafter the length of the deteriorating parts is measured. After 100 pieces of the deteriorating parts are measured, the distributions thereof are displayed on a display circuit 10. In a control part 8, the timings of the processing speeds in the respective parts are matched.