An apparatus for detecting periodic defects on a target sample, comprising: a sensor configured to obtain signals for evaluating properties of an area having a length longer than an expected defect period on the target sample; the apparatus being characterized by further comprising: a small area selection means (72, 74) configured to separate a plurality of small areas (2, 3, 4, 5) whose area length is shorter than that of the area so that all distance intervals (d) between separated small areas adjacent to one another are equal in the periodic defect arrangement direction to determine positions of the plurality of small areas, and to select signals corresponding to the positions of the plurality of small areas from outputs from the sensor; an evaluation index calculation means configured to calculate a similarity evaluation index between signal patterns among a plurality of signals selected by the small area selection means (72, 74); a set value changing means configured to change the positions of the small areas and the distance interval (d), and to repeat computational processing of the small area selection means (72, 74) and the evaluation index calculation means; a period judgment means configured to judge the distance interval (d) as a period when the evaluation index is higher than a value set beforehand; wherein the area length, the small areas lengths, the distance intervals (d) and the positions of the plurality of small areas are measured along the periodic defect arrangement direction; and a defect judgement means configured to judge the existence of a periodic defect in the small areas on the basis of numerical values of a signal strength, a length in the width direction, a length in the rolling direction, and a shape of the defect candidate in the small areas when it is judged that there is a periodic defect candidate, and, when the defect candidate is judged to be a defect, to output a result with the period obtained by the periodicity judging means. a sensor configured to obtain signals for evaluating properties of an area having a length longer than an expected defect period on the target sample; the apparatus being characterized by further comprising: a small area selection means (72, 74) configured to separate a plurality of small areas (2, 3, 4, 5) whose area length is shorter than that of the area so that all distance intervals (d) between separated small areas adjacent to one another are equal in the periodic defect arrangement direction to determine positions of the plurality of small areas, and to select signals corresponding to the positions of the plurality of small areas from outputs from the sensor; an evaluation index calculation means configured to calculate a similarity evaluation index between signal patterns among a plurality of signals selected by the small area selection means (72, 74); a set value changing means configured to change the positions of the small areas and the distance interval (d), and to repeat computational processing of the small area selection means (72, 74) and the evaluation index calculation means; a period judgment means configured to judge the distance interval (d) as a period when the evaluation index is higher than a value set beforehand; wherein the area length, the small areas lengths, the distance intervals (d) and the positions of the plurality of small areas are measured along the periodic defect arrangement direction; and a defect judgement means configured to judge the existence of a periodic defect in the small areas on the basis of numerical values of a signal strength, a length in the width direction, a length in the rolling direction, and a shape of the defect candidate in the small areas when it is judged that there is a periodic defect candidate, and, when the defect candidate is judged to be a defect, to output a result with the period obtained by the periodicity judging means. a small area selection means (72, 74) configured to separate a plurality of small areas (2, 3, 4, 5) whose area length is shorter than that of the area so that all distance intervals (d) between separated small areas adjacent to one another are equal in the periodic defect arrangement direction to determine positions of the plurality of small areas, and to select signals corresponding to the positions of the plurality of small areas from outputs from the sensor; an evaluation index calculation means configured to calculate a similarity evaluation index between signal patterns among a plurality of signals selected by the small area selection means (72, 74); a set value changing means configured to change the positions of the small areas and the distance interval (d), and to repeat computational processing of the small area selection means (72, 74) and the evaluation index calculation means; a period judgment means configured to judge the distance interval (d) as a period when the evaluation index is higher than a value set beforehand; wherein the area length, the small areas lengths, the distance intervals (d) and the positions of the plurality of small areas are measured along the periodic defect arrangement direction; and a defect judgement means configured to judge the existence of a periodic defect in the small areas on the basis of numerical values of a signal strength, a length in the width direction, a length in the rolling direction, and a shape of the defect candidate in the small areas when it is judged that there is a periodic defect candidate, and, when the defect candidate is judged to be a defect, to output a result with the period obtained by the periodicity judging means.