A method of detecting a streak extending in a first direction from an inspection image, the method comprising: an image acquisition step of acquiring the inspection image; a dividing step of dividing at least a part of the inspection image into a finite number of local regions, wherein the local regions may partially overlap in the first direction; a grouping step of applying grouping processing to the finite number of plural local regions connecting in the first direction so as to form local region groups, wherein adjacent local region groups overlap in the first direction; a streak intensity signal creating step of creating a streak intensity signal that quantitatively shows streak intensity at a position in a second direction intersecting with the first direction for each of the finite number of local regions; a detecting step of detecting a position of a streak-like signal in the second direction by applying each of at least one of T thresholds where T is an integer of 1 or more to the streak intensity signal to detect the streak intensity signal as the streak-like signal; and a determination step of determining existence or non-existence of a streak according to a detection result acquired in the detecting step, wherein the determination step is configured to determine whether there is a streak for a certain position in the second direction in accordance with the number of times of detection of the streak-like signal for each threshold in a local region group composed of two or more local regions connecting in the first direction in the finite number of local regions, the number of times of detection required for determining that there is a streak is determined for each of the T thresholds, and two or more times of detection are required for determining that there is a streak for a threshold of at least one of the T thresholds. an image acquisition step of acquiring the inspection image; a dividing step of dividing at least a part of the inspection image into a finite number of local regions, wherein the local regions may partially overlap in the first direction; a grouping step of applying grouping processing to the finite number of plural local regions connecting in the first direction so as to form local region groups, wherein adjacent local region groups overlap in the first direction; a streak intensity signal creating step of creating a streak intensity signal that quantitatively shows streak intensity at a position in a second direction intersecting with the first direction for each of the finite number of local regions; a detecting step of detecting a position of a streak-like signal in the second direction by applying each of at least one of T thresholds where T is an integer of 1 or more to the streak intensity signal to detect the streak intensity signal as the streak-like signal; and a determination step of determining existence or non-existence of a streak according to a detection result acquired in the detecting step, wherein the determination step is configured to determine whether there is a streak for a certain position in the second direction in accordance with the number of times of detection of the streak-like signal for each threshold in a local region group composed of two or more local regions connecting in the first direction in the finite number of local regions, the number of times of detection required for determining that there is a streak is determined for each of the T thresholds, and two or more times of detection are required for determining that there is a streak for a threshold of at least one of the T thresholds.