At least a part of an inspection image is divided into a finite number of local regions by allowing overlap in a first direction. A streak intensity signal quantitatively showing intensity of a streak for a position in a second direction intersecting with the first direction is created for each of the local regions. A position of a streak-like signal is detected by using each of at least one of T thresholds, where T is 1 or more. Presence or absence of a streak is determined for a position in accordance with the number of times of detection for each of thresholds in a local region group composed of two or more local regions connecting in the first direction. The number of times of detection required for determining that there is a streak for at least one of the thresholds is two or more.