PROBLEM TO BE SOLVED: To measure cracks in a form which analysis is easily, exactly, quickly and objectively done.
SOLUTION: In a crack measurement method for structure surface, the surface of structure of object crack measurement is photographed, which is made an original image data 101. Then, among image region of shading corrected original image data, whole photographing range for example, is made specified crack survey region and this crack survey region is binary-processed with a specific threshold 103. Next, crack is extracted by using the binary-processed image data 104 and crack survey region where not-extracted crack potentially exist is identified 105. For the identified crack survey region, binary-processing of the original image data is repeated with a smaller threshold than the initial threshold and new crack is extracted 106. The series of processes 105 to 106 are repeated until crack with desired accuracy is extracted.